I changed my gamertag about a year ago and didn’t really notice but I lost my gameplay stats from Halo 2 through Halo Reach. It looks like my “Halo Gamerscore” still has them recorded, but I can’t see the stats. Is there any way to retrieve it?
Hey OP - I have good news and bad new for you 
The good news is that you can recover your stats by loading up each game and playing a match / campaign / etc and it should eventually sync everything back to Waypoint. make sure you’re singed into Xbox live while you are doing this.
The bad news is that you won’t be able to get your Halo 2 stats back - this is because the original Xbox Live servers are no longer available. The only way you could get them back would be to return to your original gamertag
Thanks!