Not qualifying for PrC

[Renen]

What happens if you take a rank in Prc and later don't qualify anymore? And not one of those PrCs where you can fall or anything

[Nettlekid]

There are conflicting opinions, mainly due to lack of clear ruling on the subject. A couple of the Completes like Complete Warrior state that if a character no longer meets the requirements for a prestige class they lose all class features and special abilities of the prestige class except for the HD, BAB, and saves that they got from it, but you can regain those class features if you later regain the requirements of the prestige class.

However, while that's said in Complete Warrior, there are some cases where that MUST not be the case. For example, in the DMG there's the Dragon Disciple, which as part of the prereqs must not be a half-dragon. When you reach 10th level, you become a half-dragon. Technically, you no longer qualify for the class and so you lose all class features (including the half-dragon template). Which means you do qualify again. Which means you become a half-dragon. Which means you don't. Etc. Forever. So clearly the "lose benefits if you fail to meet prereqs" can't apply there.

There are arguments and citations for both sides, so it's hard to give a clear answer.

[Razanir]

In light of the Dragon Disciple, I'd advocate a house ruling stating that you keep abilities, but can't progress

[ddude987]

As per game rules books of non primary source that introduce modifications to rules only apply for that book unless explicitly stating the rule is meant to replace preexisting rules. Prestige classes primary source is the DMG and to my knowledge there is no book stating the rule that you must meet all qualifications for a prestige class makes that rule apply to every source book. Of course there is the argument that it does not need to state it overrides preexisting rules because there was no rule to modify.

[Curmudgeon]

What happens if you take a rank in Prc and later don't qualify anymore?
Mostly, nothing happens. Complete Warrior and Complete Arcane have rules (substantially different rules for the two books) that require you to continuously maintain entry requirements for their prestige classes. But outside of those books, you're back to the standard Level Advancement rules (pages 58-59 of Player's Handbook) where you hit step 1. Choose Class every time. So you'd need to re-qualify if you ever want to advance another level in that prestige class, but you maintain what you've got in your existing levels. You'd just need to pick a class that you did qualify for instead.