This news old pretty old actually. We knew even before Dawnguard went public that we were going to have to delete and reclaim our GamerTags because of a screw up by Microsoft.
I'm not going ot go into massive detail, that can wait for a good PM exchange. But the gist is this. When the Expansion went live, we already had the achievements from when we downloaded Dawnguard. But Microsoft didn't have the achievements up on the Achievo Servers yet, so while or Xbox profiles/GT's said that we had unlocked said Achievo, the Official version of our Profiles/GT's said that we had not yet unlocked them. So in order to make the Achievo's count, we had to delete and reclaim our GamerTags thus wiping out our Beta unlocked Achievo's for Dawnguard.
They aren't difficult to get, just time consuming. It won't be too bad of an effort to get them again.
