I got the impression reading the FAQ, you could select multiple characters but not being on one of the transfer servers, I don't know for sure.
As for what happens, its called passive incentives As your 100+ characters move over and the population dies you will find more and more reason to move and less and less reason to stay and eventually move
You might be left with a few fringe cases that stay until they are forced to move, but give it a month and most everyone will have moved that could I predict.