Once a PN has been served it is not just a case of forgetting about it. It's normal practise to carry out regular visits to check compliance. If during one of these visits it becomes apparent that a change of RP has occurred, simply withdraw and re-serve on the new RP if still necessary and then attempt to get the standard raised thereby enableing the withdrawal of the Notice, why leave in doubt the legality of the original Notice and whether or not it is still valid, the re-serving is a simple process.
Failure to comply with a PN is an offence in its own right as PhilB states quite rightly, however, it still has to pass the evidential test and the public interest test, I would suggest that any half decent brief would have something to say on behalf of his client in both of these respects should a FA attempt to prosecute someone other than the RP who the original Notice was served on.