Would like my old gamertag back..

That will only be possible if someone else doesn't own that Gamertag.

If someone does, then nothing can be done about. If the gamertag is inactive you'll have to wait for the general reset like everyone else.

Sorry for the inconvenience.

As of last year if a gamertag does not sign in for 5 years it is then made available.

That's the same for everyone,no matter how long you've been with xbox.

Hello, first it's a pleasure to be able to help you today.

My name is welcome to the community and Merry Christmas.

Hello Lennon, by your description it seems that your account was hacked, because by definition of the Microsoft system you would not be able to create an account with the same email.

You mention that he created another email when trying to log in, was the email he used the same as his old account or did you create a new email?

But the gamertag that I want to change is my old gamertag. I change my old gamertag on 2 August 2020. I used that old gamertag when I create my Xbox account. At that time, the system allow me to use that gamertag. But when I want to change it back, the system don't allow me to use it. Why is this like that?

Im wanting my old gamertag back as my brother change it and it's a unauthorized change so I'm requesting my old gamertag back

If it was an unauthorized purchase the only people that can do anything IF your account is eligible would be xbox billing support who basically IF your account is eligible would give you a credit to your account so that you can change your gamertag back
to the old one.

Again this is only

IF your account is eligible.