Prestige Classes and Spellcasting

In 3.5 there are some prestige classes that require spell casting to enter but then give you a brand new spell list, such as Knight of the Chalice. How unbalancing would it be to swap the new spell list and just advance a PCs previous casting abilities?

What if a PC enters into a prestige class that has its own spell casting but doesn't require it for entry? For instance, what if a cleric became an assassin. Could (should?) they swap out the assassin spells to advance their previous spell casting?

Do you think advancing the spell casting partially is better than full casting progression?

The answers are, in order, no and no.

You take the new class, that's your new class, whether it's a PRC or a cross-class. Unless it says you continue advancing spells as the old class, you don't.

You you want the old class abilities or spell casting, stay in the old class, or go back to it.

Sorry if this seems blunt, but you know me, my preferred mode is RAW.