I assume Anet will keep their eye on server populations since WvW is one of the important features of GW2. They have already implemented a server transfer system that seems very robust. The gem cost for home server transfers is merely there to keep people from switching to the best wvw server each week (proven by the fact that you can still play on any server you like, except for WvW) so I assume they will allow free transfers if server population issues exist.