So I changed my Gamertag and now my Halo service history wont show up in Halo 2, 3, reach, and ODST. It just says I’ve never played these games. I’d hate to lose all the hours I had invested in these games. Has anyoneelse had this problem? If so, is there a fix for it? Thanks.
Easy fix for the most part OP - if you have have the games available to play on your 360, just load each one up and play some campaign or multiplayer (make sure to be connected to Xbox Live). After a short while everything should sync back up with your new gamertag. Halo 2 I’m afraid won’t sync up becuase it relies on the original Xbox Live servers which are no longer available. They should still exist under your old gamertag, but the only way to keep all your stats in one place would be to switch back to that tag 
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> Easy fix for the most part OP - if you have have the games available to play on your 360, just load each one up and play some campaign or multiplayer (make sure to be connected to Xbox Live). After a short while everything should sync back up with your new gamertag. Halo 2 I’m afraid won’t sync up becuase it relies on the original Xbox Live servers which are no longer available. They should still exist under your old gamertag, but the only way to keep all your stats in one place would be to switch back to that tag
This explains where my previous game history went years ago, though I never got it back in the way you described. I cant even remember my first GT, some random Microsoft assigned one.