I am not sure you can add a Seaplane rating to a 61.75 but stand to be corrected on that front.
I have a 61.75 with an FAA seaplane rating (ASES). Issued when taking the rating at Jack Brown's without the need to visit the FSDO at all. I've since added ratings to my JAR-PPL which has been reissued, and believe it continues to be valid.
My *assumption* was that the 61.75 would remain valid as long as the underlying UK licence continues to have the same licence number - the text says something like "based on licence XXXX issued by UK CAA". So as along as the licence number doesn't change, then even if you move I don't see why it wouldn't be possible to retain the 61.75 (you also have to tell the FAA that you've moved).
My *hope* is that when my JAR-PPL is replaced by an EASA equivalent, the same licence number might be used. In this case, the 61.75 could continue to be valid after the transition. But until EASA licences start to be printed off in July, we may not know.
If moving to the US, then clearly getting a full FAA licence sounds like a good idea. For those visiting occasionally on business, I can't see that the hassle of arranging an M1 Visa and TSA approval etc. (which must be done long in advance of travel), is worthwhile.