Its a strange one, I'd be delighted if my FAA PPL would still be valid and be based on my JAA CPL.
Worst case scenario I'd have to say goodbye to my US flying after I get my CPL or I'd have a get a regular FAA PPL to continue flying over there.
@Mark, I was under the impression that the PPL automatically becomes invalid when issued with a CPL, I didnt realise you could have both.