How were mission presidents called and trained? I only have two examples, told from only one perspective, so I?ll simply give an ?and then? and then? and then?? narrative. Also: I think the correspondence loses more than usual in gloss, so I?ll reproduce many of the diary entries.
James G Duffin began his second mission as a traveling Elder in 1899 October. He was almost forty and left a wife and several children in southern Utah. If he anticipated being called as a mission president, I haven?t noticed it in the diary. 
Five months in Duffin was called as a Conference President  and one month later was told to report to mission headquarters ?to assist in the general duties? of the mission.  The morning after his arrival ?Prest. Wm. T. Jack, read a letter to me? notifying me that I had been chosen to succeed Prest. Jack?.?  If the calling surprised Duffin he did not write it in the diary. 
Duffin and Jack spent four days together working on mission business and ?in the office getting acquainted with the office work? then Jack left to visit Mormon colonies outside the mission.  Two weeks later Duffin assumed full responsibility and four days after that Jack left. 
Duffin himself was released in 1906. The First Presidency asked Duffin about his malaria; he reported early in July that it made him ?wilt? in the summer.  The return letter informed him that ?if they [the First Presidency] can find a man capable of taking your place, they feel that it will be right to release you.? The letter also asked for recommendations on a replacement. 
Duffin?s letter could be read as an oblique request for release, but Duffin ?was a little surprised? at the outcome.  A possible factor in the decision was Duffin?s polygamy (since 1902). As the Church attempted to leave polygamy behind, many polygamist leaders were quietly released. 
Duffin recommended Samuel O Bennion and gave three other names of ?capable? Elders; he also suggested delaying the transition four months and requested permission to go home for the harvest.  The return letter accepted Duffin?s requests and further suggested that ?While you are away you could leave Bro. Samuel O Bennion in charge, to accustom him to such duties, but without saying anything to him as to the possibility of his succeeding you in the Presidency.? 
In late September Duffin met with the First Presidency in Salt Lake City and ?they decided to appoint Elder Samuel O. Bennion.?  Duffin did not record how or when Bennion was informed of the decision, but Bennion and Duffin traveled to conferences together at least from 1906 Oct 14 to Oct 28, which is Duffin?s last entry.
The ?Southwestern States Mission? series uses the diaries of six missionaries who served in eastern Texas around 1900 to illustrate aspects of Mormon material culture, lived religion, and social History. The missionaries are Mission President Duffin and Elders Brooks, Clark, Folkman, Forsha, and Jones. The series is inspired by Ardis Parshall?s serial posting of the missionary diary of Willard Larson Jones at Keepapitchinin. Previous installment here.
 Being ?older? and on a second mission was probably not, by itself, enough of a clue. Although most of the Elders were in their early twenties, in 1899 it had not yet become unusual to call men in their thirties or older on missions. Further, when President Duffin was replaced six years later, it was by a traveling Elder, but I haven?t noticed any indications that President Bennion was called as a ?replacement President? rather than just an ordinary traveling Elder.
 Duffin, 1900 Mar 04 Sun. ?Conference President? would be, loosely, ?Zone Leader? in present-day mission organization. Duffin was called when the previous president completed his mission and returned home.
 ?This epistle will inform you that you have been chosen to assist in the general duties of the South-western States Mission, therefore you will be located at this office, to which point you will kindly proceed at your earliest convenience. You may feel sorry and disappointed at this unexpected call from your conference and successful labors, but we trust that you will see the providence of the Almighty in this change? (Duffin, 1900 Apr 6 Fri). Of the new assignment, Duffin wrote: ?May my Father in Heaven strengthen me and give me wisdom to perform every duty in this new calling acceptably to Him.?
 Duffin received the note assigning him to the mission office on Apr 06; the letter of appointment as Mission President was dated Apr 07; he arrived at headquarters on Apr 15 and learned of the letter on Apr 16.
As transcribed by Duffin, 1900 Apr 16 Mon: ?Following is the letter of appointment. Office of The First Presidency of the Church of Jesus Christ of Latter-day Saints P. O. Box 13. Salt Lake City, Utah. April 7th 1900. Elder James G. Duffin, Dear Brother, Elder Wm. T. Jack having been released from his labors as President of the Southwestern States Mission, you have been appointed to succeed him. It will now become your duty to take charge, as President of the Mission, of all the interests connected therewith; to see that the Gospel is preached, as far as possible, throughout the States where the Elders now labor, and as the Lord shall open the way, to seize any new opportunities which may present themselves for the introduction of the Gospel to regions where it has not been preached. In short we desire you to go forth as a servant of the Lord Jesus Christ, fully equipped for the discharge of every duty connected with your calling, and to preside with wisdom and dignity over all the affairs of the church in those states, with full power to regulate everything connected therewith, and to make such changes and appointments as may in your judgment, under the direction of the Spirit of the Lord, be necessary for the welfare, spread and prosperity of the work of God and the warning of the people in the field where you labor, and over whom you are appointed to preside. And that you may be fully sustained and qualified for these high and responsible labors, we beseech God our Eternal Father to endow you with power from on high, and to fill you with his Holy Spirit and all the gifts that pertain to your office and calling, that you may be a blessing and saviour to the children of men in your ministry and carry with you an influence and power that shall be felt for good by all with whom you are brought in contact. It is our most earnest desire that you should be an example in humility, in patience, in longsuffering, and in all the gifts and graces of the Gospel, to all men, that your words may be the words of the Lord to the people whom you address, and that you may feel that there is a power and spirit accompanying you that is not of man. We shall be glad to hear from you concerning your labors and the situation of affairs in your field of labor as often as it shall be convenient for you to write; and shall esteem it a pleasure ourselves to communicate frequently with you. Praying God the Eternal Father to bless and qualify you for the work to which you are now called, and asking him to preserve you and yours from the hands of all your enemies, and from every evil, during your absence, and bring you home in purity, peace and safety, We remain, Your Brethren in the Gospel, Lorenzo Snow, George Q. Cannon, Joseph F. Smith.?
 Duffin did express some thoughts on the call: ?This is a very great responsibility placed upon me by my brethren in the Priesthood, Prophets of the Living God, but I pray my Father in Heaven that he will qualify me for every duty pertaining to this holy and responsible calling in the work of the Lord. My desire is to do all that lies in my power to bring souls to a knowledge of the truth, and to build up the work of the Lord in this dispensation.? (Duffin, 1900 Apr 16 Mon). That same day, Monday, President Jack ?made me a present, from the mission, of a fine Prince Albert suit of clothes.? I don?t know anything about the motives for President Jack?s release or why his term was so relatively short (Kimball, 12; Jack, 3; Duffin, 6; Bennion, 20+).
 Duffin dated his acceptance letter on Friday after learning of the call on Monday. ?Letter to the First Presidency, St John, Kansas April 20, 1900 Lorenzo Snow, George Q. Cannon, Joseph F. Smith, First Presidency of the Church of Jesus Christ of Latter-day Saints. Dear Brethren: – Your letter of 7th inst. appointing me to succeed Elder Wm. T. Jack in the Presidency of the Southwestern States Mission is before me. I entering upon the duties of this high and holy calling, I do so realizing my dependence upon my Father in Heaven for wisdom to guide me in the performance of those duties. It is my constant desire to advance the work of the Lord in this mission; to this end I shall endeavor to be directed by the whisperings of the Spirit of the Lord, for I know without that I must fail. While sensing the grave responsibilities thus placed upon me, I feel grateful to my Father in Heaven that I am considered worthy to fill so important a position in his work; and my prayer is that I may be an honor to his cause. It will afford me pleasure to receive communications from you at any time, and instructions for the advancement of the work of the Lord in this part of his vineyard. In reviewing the labors of Elder Jack, I feel that he has done a work that is a credit to himself and to himself and to the cause of truth. In leaving the mission he takes with him the love and confidence of the Saints, and the Elders who have labored under his Presidency. Ever praying for the welfare of Zion, With kindest love, your Bro. in the Gospel James G. Duffin.? (Duffin, 1900 Apr 20 Fri)
 “Today, according to our arrangements, I enter upon the duties of the Presidency of the mission. This is an important day in my life, for I feel that if I am faithful and successful in my labors as President of the Southwestern States Mission, it will be the means of opening before me a wider field of usefulness in the work of the Lord. I feel to humble myself before my Heavenly Father, and pray for His spirit to help me to be faithful in discharging the duties of this responsible calling? (Duffin, 1900 May 1 Tue).
Three days later ?we gave President Jack a nice reception at the Church, During the evening I presented him a fine gold watch and chain, in behalf of the Elders laboring in the mission. It was greatly appreciated by Prest. Jack.? (Duffin, 1900 May 5 Sat) and then, the following day, ?Prest. Jack took his departure for home, with the blessings of the Lord upon him for duty faithfully done.? (Duffin, 1900 May 6 Sun). Duffin made no diary entries for the subsequent three weeks.
 As transcribed in Duffin, 1906 Jul 17 Tue: ?Kansas City, Mo. July 7, 1906. Elder George Reynolds, Salt Lake City, Utah. Dear Brother: Replying to your inquiry concerning my health this summer, will say: I get along pretty well when we have cool days, which we are having at the present time, but as soon as the weather changes to the heat that we experience here, I “wilt” under it. It seems that the many years spent in malarial states have filled my system with malarial poisons, and the heat of summer develops that tired, languid, feeling that is the invariable result of this condition. However, with this exception and the trouble I have had in my back for the last year and a half, my health is good. I had thought I would take a month in Canada this summer, had it been agreeable with the feelings of the Presidency, but my presence is so badly needed in the mission during these months that are so trying to all of the Elders, that I feel that I must put aside that which would be for my own good, so as to be with my brethren during the time I am most needed. My constitution and temperament demand a cool climate. Brother Reynolds, I have always felt that I did not want anything to interfere with my service to my Master in the mission field, and that when I was released, it would be because my brethren of the priesthood had accepted our offering, not because of ill health, or for any other cause. My family all feel the same way, and should a release come in any other way it would be a great disappointment. We feel thankful for the blessings we receive from the Lord and for the good will and confidence of our brethren, and when we are released we hope to be of some service in building up some industries at home, as well as doing what we can in a spiritual way. With kindest personal regards, I am, Your Brother, James G. Duffin.?
 As transcribed in Duffin, 1906 Jul 17 Tue: ?Salt Lake City, Utah. July 14, 1906. Prest. J. G. Duffin, Kansas City, Mo. Dear Brother. — The First Presidency have read with much interest, your form to me of the 7th inst., and they direct me to say that you have been long and faithful in the service, and if they can find a man capable of taking your place, they feel that it will be right to release you. And they wish me to ask you if there is any such a one in the mission at the present time, whom you could recommend to take charge of the mission, or if you know of any one who has labored there in the past who is competent to fill the place. If there is either of these, they would like you to let me know, so that they can give the matter their early attention, and if a brother is found who is suitable to appoint him in your stead, and give you an honorable release. Sincerely your brother, Geo Reynolds, Sec.?
 As transcribed in Duffin, 1906 Jul 17 Tue: ?Kansas City, Mo. July 20, 1906. Elder George Reynolds, Salt Lake City, Utah. Dear Brother, — Your letter of 14th inst. came to me in due course of mail and has received careful consideration. I was a little surprised to learn that the Presidency had decided to release me from my labors in the mission, but am very thankful, indeed, that inasmuch as this has been decided upon they feel that I have been faithful in my duties. My labors in the mission have been a source of satisfaction to me, for I have loved the work and my brethren among whom I have labored. ?
 I didn?t chase down a reference; I think the point is commonly accepted. Note, however, that I have found no evidence in the diaries to implicate polygamy in Duffin?s release. The only things that might even be hints in that direction are Duffin?s surprise at the release and a line in a subsequent letter from Reynolds (First Presidency secretary): ?There are reasons why I cannot write more definitely at this time, but that can be done when you see the brethren at the time you make your visit.? (Duffin, 1906 Jul 17 Tue) ?Reasons? in the first decade of the 1900s often, but not always, turned out to be ?polygamy.?
 As transcribed in Duffin, 1906 Jul 17 Tue; paragraphing mine: ?Now concerning those who, in my judgment would be suitable to take charge of the work when I am released: Elder Samuel O. Bennion, now acting as secretary of the mission is an Elder whom I can recommend with a full assurance that the work would be safe in his hands. As a traveling Elder, as president of a Conference, and later as secretary of the mission, he has been faithful, energetic and capable. He is true to the priesthood, loves the work and has never shrunk from any duty to be performed, nor complained at any requirement made of him since he has been in the mission. Elder Jasper Newbold who recently came into the mission, I believe would make a capable presiding officer. I cannot say anything about his work for he has not been tried as yet. I have observed this, however, in Elder Newbold–he has manifested a willingness to do whatever is desired of him. He is now laboring in St. Louis. There are two other capable young men in the mission, Elders Osburn Richins, of Mesa, Arizona, and Henry J. Bodily, of Fairview, Idaho.
?I would like to suggest a matter for the consideration of the Presidency in deciding on an Elder to take the presidency of the mission. There are two branches in Kansas, at which we have property interests, our colony in Texas, where we have secured property for the church, and there is the property at Independence. I have been working to get the titles to one piece of property in East Kansas and one in Texas completed. It is but recently that the way has been opened for this to be done, and I feel that this ought to be finished before I leave the mission. ; Another thing, during this month and until the middle of next month is the most trying time of the year on the Elders. It is during this period that we have the most of our sickness. It would be very trying to one just coming in to have these conditions to meet.
?The suggestion I have to offer is this: that I continue in the mission until after the October conference, and then go through the mission with the new president, visiting the conferences, during with time I can get the property titles completed, and he will become a little better acquainted with the mission. However, if the Presidency feel otherwise it will be agreeable with me.
?One thing further, should the Presidency see fit to adopt the above suggestion, if it is agreeable with their feelings, I should like to go to my home the latter part of August, to assist the boys in shipping our crop of fruit. During that time we employ between forty and fifty hands and some one is needed there to look after the business. With kindest personal regards, I am Your Brother, James G. Duffin.?
 As transcribed in Duffin, 1906 Jul 17 Tue: ?Salt Lake City, Utah, July 23, 1906. President J. G. Duffin, Kansas City, Mo. Dear Brother;— Your letter of the 20th inst. has received the considerate attention of the First Presidency, and they entirely agree with you that the conditions are such in the Central States Mission that it will be well for you to remain at their head until next October. They most cordially grant your request regarding paying home a visit at the time of the shipping of your fruit crop. While you are away you could leave Bro. Samuel O Bennion in charge, to accustom him to such duties, but without saying anything to him as to the possibility of his succeeding you in the Presidency. There are reasons why I cannot write more definitely at this time, but that can be done when you see the brethren at the time you make your visit. Trusting your health remains good, I remain, as ever, Sincerely your brother, George Reynolds, Secy.?
 ?To-day I went up to Salt Lake City, according to appointment, and met the Presidency. Prest. Smith had returned from Europe a few days previously. Had a talk with the Presidency with regard to the appointment of someone to succeed me in the Presidency of the mission. Of the names I had suggested, they decided to appoint Elder Samuel O. Bennion, of Taylorsville, who has been acting as secretary of the mission and he was so appointed.? (Duffin, 1906 Sep 24 Mon)