They did that in DOS2 did they not?
I really liked that since i could keep the character but change the class. I think we would need to keep closer to the original role, like you said, since some stories are closely tied to the some class features. Having Gale as a fighter would ruin his story a little. Wyll could be problematic since no other class i
know has a Demon Pact. Not even a Star Pact would work in this regard. You would have to untie Mizora from his class.