There’s no migrating involved. ALL of the regional servers pass data back and forth with ONE GLOBAL database. So even when US or EU servers are low-traffic, they’re running on a queue because of Asian prime time database traffic making it hard for them to connect.
Supposedly the engineers are busy recoding things so that fewer database connections are called for every time someone logs in, creates a game, etc. But there’s no transparency about the progress they’re making or the timeline they’re working on.