TMFKA - the only change to the regs as far as an existing FAA license
issued on the basis of a foreign licence, is that you can no longer add any further FAA ratings to it - it is now frozen at whatever qualifications you already have on it.
All - don't despair on the USA hour building - the FAA examiners are all discussing the question - all you need to do is a simple written exam (which you can even do in the UK before you come out, if you wish). Read
the private pilot test questions - this is the FAA PPL Confuser.
When you get to the states - you will have to do a school check out anyway....turn this into a few hours of training (you are trying to "hour build" - get it done with a dual rated [FAA & JAA] instructor and it will count for your JAA PPL renewal flight as well - there are schools, certainly in Florida, that have these people) and then do a flight test with an FAA examiner.
You will have a full blown, stand alone FAA licence and can do much, much more with it in the future.
Don't turn away because of a minor hurdle - it came as a shock - but it is no big deal.
The biggest problem is that, until the relevant FAA office publish their guidelines, each individual examiner may well have their own idea of what experience/training you must have to fly with them. This will be down to the individual flight schools to check with their local DPE's.
Ask your intended aircraft rental outfit - get it in writing - "What will I have to do to get an FAA PPL with your local DPE?"