Question for Torrent and Steam users.?

I downloaded The Elder Scrolls V: Skyrim off of the Pirate bay and I must say that It is an amazing game. I Got pretty far in it, Im at about level 30+, and tomorrow I'm buying it off of Steam because there was a price drop and I want to take advantage of it. Now there's one thing I actually care about and that is the Steam Achievements. Now obviously I can't get achievements on the torrented version of Skyrim and that's fine with me but the thing that I'm most worried about is this...in order to get my achievements from steam, would I just need to transfer my saves and run Skyrim on Steam and them Steam would recognize all the achievements I could have won? Or would I be doomed to restarting the game so that steam could register the achievements as I unlock them? That is all, I hope to hear the best.