I've moved a fair few lots over to new world with a 99% success rate xD I think the largest issue is those who have 31x31 lots, they don't fully fit in a 30x30.. Although I kinda just cut off part of it in most cases because they havent built on that edge.
As for the warning, thats a decent idea, although by the time we update, it might be much closer to the time that we remove the world..