My apologies, but this is an indication that you absolutely must use the world transfer and failing to do so will randomize you into a world, so...
It means there is no option to 'do nothing to stay exactly where you are' correct?
In that case it's a very good idea to have the transfer open for a long period of time as mentioned, I remember the last set of transfers and how very quickly the 'popular' worlds would fill up...
Of course based on local server locations this could change a worlds popularity
/standing by for more info