I could have sworn I read somewhere that you can't retrain feats that are prerequisites for other feats (that is, they are a prerequisite for a feat you took at a later level or are a prerequisite for a PrC).
However, re-reading the rules the only rule that even resembles it I can find is the rule on changing class-benefit decisions that cannot be changed if they make later choices illegal.

Any solid ruling on this?

Or does retraining a prereq rule simply render the depending feats and PrC useless, just as when a prereq ability dropped, a ranger wears heavy armor (and nullifies his combat feats) etc.