Starting from lvl 1? I honestly hate this idea. We have current vanilla classic to lvl up toons, if TBC drops I personally sure as hell don't want to be forced to re-do the content I was doing in the past year(s), depending when TBC drops, just so I can get to new content (yes, even with the nerfs vanilla areas received).
I'm also pretty sure most people would feel the same way, I don't see this happening. Imo it'll either be copy your toon to a TBC realm or the other way around, copy it to a permanent vanilla realm as current ones progress to TBC.
A bat in the hand is worth two in the belfry.