Good question, that. It’s been a while since I looked, but it seems to me that the original announcement avoided the “God is changeable” problem by suggesting, somewhat obliquely, that while the darkness of tone had always corresponded to a less-good spirit, this correlation had attenuated over time to the point that a distinction, for purposes of eligibility to the priesthood, was no longer appropriate.
It seems to me, too, that some of the basis of this whole Mormon race theory came from the secondary writings which have caused the Mormons some embarrassment, particularly the Pearl of Great Price, significant parts of which were “translated” from Egyptian papyri which we now can see the actual translations of. The Mormons were thought to have had good luck on this score, those papyri having burned up in the Great Chicago Fire, but then the doggoned things turned up. Someone noticed that a document with a headless Anubis had a sort of Mister Magoo head sketched in on the backing paper, and put two and two together, and the whole thing came apart. But nowadays the Mormon church tries to get you not to read the Pearl of Great Price, and the missionaries I had at my house were entirely unaware of some of its contents.
But the BoM itself contains sufficient references to skin tone corresponding to moral character to be quite embarrassing: “white and delightsome” is how it is put in at least one case. And there are, if I recall correctly, explicit references to people’s skin tone getting darker as they drift into immorality. Quite a document, that Book of Mormon.
Yeah. I think that the function of these missions, to a great extent, is actually to train them to do just that. There’s hardly any success at getting converts, but the combination of being forced to defend the faith against all objections and the sunk-cost fallacy that keeps people in the faith after they’ve gone on these absurd missions does seem to work, at least on some.