Actually I believe this has happened quite often (not that I'm going to study it). Three examples in American history come to mind, Hawaii and Texas statehood, both due to American emigrants coming back to America. Then, West Virgina, didn't want to join the Confederacy so they broke away from Virgina and came back to the Union.
Again I haven't sat down and studied history for a long time.
In GalCiv I population was very abstracted. When a switch occurs I'm sure there is a good deal of another civ's population living there. That's how I always thought of it.