The divorce would be easier than you think. You put a deadline on citizens in every region to get their affairs in order and move or accept their new passport for the country that it will become. Say, 10 years. Everyone gets 10 years to decide whether they are staying or going before the walls go up. Would there be some chaos? Sure, a little. And it would take some time for the dust to settle internationally, but life will go on. No biggie.