How to change classic gamertag without an Xbox 360?

The old 360 gamertag is now considered a "classic" gamertag. You would need to change your tag again. If it gives you the option to change the classic tag, you would only change that one. Or you would change it on a 360.

I'm trying to change my classic gamertag which is the one you see in games. My classic gamertag got changed and my Xbox One and Xbox App gamertag stayed the same (the Xbox One gamertag is the ones they added with the new gamertag system).

My Xbox One gamertag is different to my in-game gamertag and I want my classic gamertag to be changed. You can only change your classic gamertag on the Xbox One console, I have tried using the other ways and you can only change your Xbox Gamertag, not your
classic gamertag.

The reason I want to change my classic gamertag is that the classic gamertag can be fifteen characters and the Xbox One gamertag can only be 12 characters. If I use the other ways to change my gamertag it limits me to 12 characters and sets both my gamertags
to that.

So you said I can change my classic gamertag. How do I change that after already changing my gamertag or do I have to change my entire gamertag again to change the classic one?

It's because you have a "classic" gamertag and a new one (as seen in your last photo). With the new gamertag policy, some people have both. It sounds like you changed the new one, but not the classic one. So the classic one will show up in areas that haven't been updated to work with the new gamertag policy which is why you're still seeing that one in places. You would have to change your tag again and see if it gives you the option to change the classic tag or change it on a 360.

You would have to go to http://enforcement.xbox.com to submit a case review. If the enforcement team overturns their decision, you will be allowed to change it.