Is there a rule preventing an "arcane" caster character from transitioning to a "divine" casting career or vice versa? Keep in mind I'm only asking about the actual rules here. While I haven't read all the setting info yet, it seems clear enough already that even if there are no Imperial laws forbidding it a wizard would probably never want to join an imperial cult, and most priest probably at least secretly suspect that arcane magic may be evil/untrustworthy. But at least so far, I haven't seen any rules against it. Granted, I would probably forbid/heavily discourage it in my own game, but I'm wondering what the RAW are on this issue.

I believe that is one of those unwritten rules, mind you it may be written, I just don't recall ever seeing it.

From a purely mechanical standpoint there is nothing preventing it; however, as the FAQ points out, leaving a "casting" career from a setting standpoint is highly unusual. If it is already highly unusual to leave one of these careers for any other career to begin with, it should be even more so for going from one to the other.

mac40k said:

The Amethyst Order and the Cult of Morr are the most amicable. However, I think it hasn't been possible to be both arcane and divine since 2nd Edition started. As far as I know it's unstated, but the fluff is otherwise clear that the two sides are exclusive. Wizards are meant to be respectful of the cults, no more.