Looks like Blizzard is going to try to fix this with free character moves:
This could be equivalent to server splitting if each oversized server gets its own destination, so I hope they’ll do it that way. If they permit transfers to any servers, or even transfers from any of the oversized servers to any low pop server, there will be naming collisions as people with the same names from different servers try to transfer to the same destination.
My memory is that free transfers were not that successful in Vanilla, but I could be misremembering. However they do it, I hope it works, and I applaud Blizzard for continuing to try to fix the problems caused by an inadequate number of servers at release.