I imagine they'd seed a few million profiles to start with, so probably a few people ("a few million" equates to "a few" in the Facebook ecosystem!) got converted over to hotstart things. Not surprising, really.

Allowing you to change back to your old profile at will wouldn't be a very feasible move, I'd imagine. You want to move your userbase over to the new code as soon as possible so you can start iterating on that for everyone, without worrying about maintaining two versions.