As soon as you do not have the prerequisites for something, it does not work anymore (you still have it but it has no effect), if it is the first feat of a feat chain, all the feats from the feat chain stop working.
if you use it to qualify for a prestige class, you lose all the class feature, including the +1 caster level in a previous class.
You however keep the hit points, the skills points, the Bab, the save bonus and the global character level.
Rule:
http://www.d20srd.org/srd/feats.htm#prerequisites