Why would you want a FAA PPL based on your JAA PPL unless you intend to fly in the states?
To fly an N-reg plane worldwide. With a UK/JAA license one can only it in the UK only.
Unless there were changes made in the last 5 years, the US FAA never validated foreign Instrument Ratings-- if you wanted an Instrument Rating with that you had to submit yourself to the written and practical exams in the United States.
I have no personal knowledge of this but a friend has just looked into this and found out something very similar. One exam and (IIRC) a checkride.