What do you mean, evade the issue? That IS the issue. Maybe I'm misunderstanding what you're asking, so I'll try again.
We have a player that wanted to play a cleric who stole spells from the gods instead of praying for them, so we refluffed the cleric so that was true from level 1. Level 6 rolls around, and he wants to take levels in Ur-Priest. Correct so far?
So, I would ask him why. If he answered with any of the three reasons I talked about, I would proceed as I said above. And if his answer was "I dunno, I just want to" then I would discuss his character's background, motivations, and personality until he understood enough about his character to be able to give a more coherent answer... and then I would proceed with one of the above methods.
You can't say "no houseruling" because that's what refluffing is, so if that's built into the premise then houseruling is acceptable. And retconning is built directly into the RAW, via rebuild quests, as I said.