FAA seemed entirely happy with my UK issued JAA PPL becoming a CPL, and my home address changing several times.
The oddball proviso was that they wouldn't re-issue the plastic card with my new address on, without going through the whole security / FSDO process again. So the FAA have all my current details and are quite happy about it, but my 61.75 licence still has an old address from about 2003 on it.
Seems to work.
I have had an FAA high performance rating added on, using a logbook entry. I'm no expert in FAA procedures, but the FAA CFII who signed it off seemed entirely happy to do so, and then to rent me his aeroplane on the back of it.
No idea about what will happen post EASA. I agree that the licence number looks likely to be the critical factor - if we can keep our numbers, all should hopefully remain peaceful and happy.
G