I'm hoping that we'll be able to lock, and stay locked at 19. I don't see why we wouldn't, turning on xp for veteran accounts would be the same as unlocking xp for every twink that lets his subscription lapse for one second. While this sort of A-hole move isn't beyond Blizzard, if they do that, they might as well just remove the ability to lock xp, for all the headaches, and tickets, it's going to cause everybody.
Beyond that, I plan on keeping my current f2ps the way they are, to troll the 29s and whatnot. If I make a p2p 20s, I'll just drag-and-drop some new ones. Though, if the whole 19s thing works out, I don't see how p2p at the bottom of the 29 bracket would be anyway superior to 19s, especially if enough people do it, which I'm sure they will, if vet accounts are able to.
What really intrigues me though, is what Latin America is going to do, since the jajas are the real keystone species in this bracket.