[Jack Simth]

With feats, it's explicitly spelled out; if you no longer meet a prerequisite, the feat doesn't function. Which means Ray of Enfeeblement is really, really mean to anyone relying on Power Attack.

With PrC's, it's not spelled out (at least, not 3.5 Core; 3.0 spelled it out, and 3.5 Complete Adventurer spells it out, but then such PrC's as the Dragon Disciple stand as nifty counterexamples).

However, part of what balance there is in PrC's is that their prerequisites pose an opportunity cost (feats, skill points, et cetera).

So, for my gameworld, when a pre-requisite for a PrC is torn from you (e.g., a Blackguard is hit with a Ray of Enfeeblement, and loseses access to Power Attack) you keep class features, but can't advance in the PrC until it's restored.

If you voluntarily lose a prerequisite, you lose class features until it's restored.

If the PrC itself causes you to lose the prerequisite (as in the case of a 10th level Dragon Disciple) you're fine.

In other words, no Limited Wishing away the now-useless feat.