James Gasik
We don't talk about Pun-Pun
Well yeah, obviously, the DM can allow for this by stepping outside of the rules! It's like how many 3e DM's were allowing for retraining before there were official rules (and those rules were ignored because they were too convoluted and messy). Sure, just abandoning abilities you've had and "magically" replacing them with new ones might seem strange, but it's not like the game didn't already have these sorts of headaches with level draining or (shudder) reincarnation!3e's additive multiclassing (and 4e's and 5e's, for all that) is one of its worst elements. 2e, where a character's classes could and did advance independent of each other, is far more flexible and ends up with non-borked characters.
I don't know if they're our own invention or whether they were inspired by a Dragon article or somehting, but for ages we've had rules and procedures in place for if-when a character wants to (and is otherwise able to) pick up a second class; meanign the decision can be made during the character's played career.
Good point, bad example: a deity could bestow the Cleric class on that character on the spot if it wanted to!
But even then, partly by 1e RAW and partly by fiat covering rules that simply don't exist, it's doable without divine intervention.
The part that doesn't exist in the RAW anywhere (and as far as I know, in any edition!) is ways and means of dropping or renouncing a class or classes other than losing all your levels to a level-drainer - and somehow surviving this process! - and then declining restoration. So the DM would have to handwave this part somehow; for my own game I invented a high-level Clerical spell Renouncement, for just this purpose.
But after that, RAW comes to the rescue. In the PH, when determining a character's age there's a modifier - you add a number of years based on the character's class: 3 years for Cleric, 8 years for MU, etc. Thus, we now know how long it takes to train in order to become a raw 0-xp 1st-level in said class; which means that in this case, three in-game years from now you could be a brand new 1st-level Cleric. The only sticky one is Fighter which adds 0 years to rolled age, but as all classes get some combat training anyway I'm cool with it only taking a few months for any adventurer to become a raw-1st Fighter.
OTOH, I'd rather my hypothetical Fighter/Thief didn't have to suffer from amnesia to gain his Cleric powers. I do remember the Complete Book of Elves discusses the possibility of allowing Elves to dual class instead of multiclass in 2e, but, well, there's a lot of issues with that book and, well, I can count the number of actual dual-classed PC's I've seen...
Ok actually I don't even need a hand. The number is three, and one didn't play for very long afterwards.