I tested this after AlanP's reply by recovering my profile and all of the acievements I unlocked during the beta were, in fact, gone. So, I guess it's not that big a deal if I can just re-do them. I was planing to replay a few times, anyway. The only thing that chaps my ass about this is unlocking the Legend achievement. I've already finished DG on my two above 70 characters, so it's going to be a grind to find a Legendary Dragon.
No offense, but why on earth would you play a new DLC (and a beta version, no less) without keeping a backup save from before starting it? Is it your first Bethesda game?

Or did you just level up a lot while playing the new content? EIther way, sorry to hear it.