I think it's implied in the rules that you cannot (the PrC says, "you advance as if gaining a level in X" whereas the class says, "You may no longer gain levels in X", and there's no ifs ands or buts about it), and I'm sure any DM who knows what he's doing wouldn't allow it without a good reason. After all, if a Cleric of Pelor becomes Chaotic Evil, and thus gets all of his powers revoked, why should he suddenly be able to use them again when he takes the Radiant Servant of Pelor class? Didn't Pelor just kick him out of his circle?