Xbox is now trying to segment their gamertag just like how Discord does. You can now register a gamertag of your choice (up to 12 characters) and if it’s already claimed by someone else then they will assign a suffix with numbers. This is to differentiate you from other users with the same name.
We entered our desired username “GameThem” here and we were assigned this gamertag with a unique ID here: GameThem#4211 or GameThem4211.
You can use this link to visit their page and change the gamertag. Just remember you can also keep your old username if you like that, there is no need to do anything. You can change the username for free once and after that you have to pay $9.99 USD for every change. Interestingly even PlayStation announced this online ID change recently. Have you changed your username aka online ID aka gamertag? Let us know!