BF2S Forums About Special Forces...

I think chuy, with his inside know-how and all, has said it'll most definitely be the same accounts. Given it's an expansion, and not just a game based on the same engine, it should be expected that it'll be with the same accounts.

It certainly does use your same account and apparently it unlocks the new ranks aswell (and maybe a new unlock or two). There is a quote on the official site somewhere, that says something like "players will continue to rise through the ranks with continuous player growth" or sumat like that.
The only thing i want to know, is are they going to lower the rank requirements again in order to incorporate the higher officer class ranks, otherwise they will be unachievable for everybody but blazin, broggi and anubis.