I changed my gamertag from Classy K1lls to Classy Melody last night and saw that my top rank (Champion #9 in BTB) is gone, and my top rank now is “Unranked.” Why? Just cuz my gamertag changed doesn’t mean my past games have just been erased. I still have the same Spartan rank so why not top arena ranking?
something similar happened to me and my company and friends. our highest level peoples all time top csrs now say unranked i posted a forum post asking about the same thing.
we did not change our gamertags however so i think it just may be some sort of glitch in the ranking system on 343s part
> 2533275011916457;1:
> I changed my gamertag from Classy K1lls to Classy Melody last night and saw that my top rank (Champion #9 in BTB) is gone, and my top rank now is “Unranked.” Why? Just cuz my gamertag changed doesn’t mean my past games have just been erased. I still have the same Spartan rank so why not top arena ranking?
This problem needs to be addressed. For now, you should try and contact 343 or Microsoft.
Like the person above said, there seems to be quite a few people who had their all-time top CSR reset when the season changed to April, so I’d say it’s more likely to do with that than your gamertag change. On a side note, my all-time top CSR has been stuck since pre-season without updating (went above it in pre-season, February and March with no update!). However, requests for assistance since then have been ignored, so I wouldn’t get your hopes up on it being fixed 
There’s a thread in the support section about this already.