I recently changed my gamertag (same XBL account). Waypoint is showing that I haven’t played Halo 3 or Reach which isn’t the case. When I search my old gamertag, Waypoint says they can’t find the user. Anyone else experiencing the same issue?
Have you loaded up any of the games since you made the switch? I’m sure I have seen a few folk post that if you load up and play a game, it should sync everything up with the servers. Worth a try at least 
edit
Also I think you meant to post this on the support forums maybe and not Spartan assault 