What I am getting out of this is those that do nothing will remain in the same world grouping. So I am kinda guessing each server will stay "as is" but not since they will be new servers but the same people that there was on the old server they were all on together, people who move will be on the server of their choice. (excluding new player servers unless starting over)
I also could be wrong, but it's what it sounds like in that clarification, I don't think it will be all one server because it would be overflown with all those who do not move, I am guessing divided based on their previous server and people choosing a server would then add to these existing amounts.