Hmm, out of curiosity, are there any prcs that are based on 'modifiable' stats (ie, Str -etc)?
I had thought they were primarily feat and skill rank based. Although I suppose Feat v Virtual Feat (ie, PrC requires TWF) or Class Feature requirement that's losable (Monk ability).
Not sure specifically on Cavalier (Sword and Fist?), so can't comment.
I'm also in a difficult position as to how anyone can lose a feat (Virtual Feat/Class ability, yes). Only a handful of items give 'feat-like' abilities (and even then, they don't provide the feat itself).
And it sorta makes sense that if someone takes a PrC that requires "Lay On Hands" Ability (or other similiar class feature for paladin), that they no longer hold benefit from the prestige class if they fall as a paladin.