I believe this is where a certain ANSP kind of screwed up their foreign controller push 10-15 years ago. There was no real gap analysis done on a controller's skills or portability of license. Just because a controller had a rating in one country, it's not a sure thing he/she will validate in another.
And, harsh truth, the motivation behind a move needs to be ascertained. IMHO and from those I have met, there are three types of controllers who change ANSP - those who genuinely want to move to a new country as an adventure, are a pleasure to work with and never say "back in wherever we did things this way"; those lured by green grass and cash; and then there are those who their previous employer and co-workers are so happy to see leave and let bridges light their way.