I have managed to hang onto my UK ATPL and convert it into a JAR ATPL while working overseas, but it has been a struggle. It was actually the CAA with their interpretation of the JAR FCLs which were a problem. JAR FCLs said I could keep my old UK rating (with restrictions), the CAA said "no". But now EASA FCL says "yes" and this is enshrined in EASA FCL and the European legistlation which enables it.
I don't have a desire to return to Europe. It's marginalising itself, especially with it's common currency which is rapidly heading towards "Andrex" staus. But my worling ATPL is a validation of my original CAA ATPL, and I have to keep that (or replacement) current if I want to move location for a future employment and another validation.
The notion that I have to take a load of exams on nonesence such as lattitude nuts in a DI because i haven't had a JAR IR for 7 years is nonesence. I have a perfectly good IR from an authority which is at least as demanding as the UK CAA, and I renew it every six months. It's like asking a doctor to retake his Biology GCSE on return to the UK after working elsewhere for 7 years.