Someone accidentally changed my account name.

it maybe one of your friendas who has changed their name, or you accidentally friended someone.

Unfortunately, you can't because of the new name restrictions, It may change in the near future. so keep your eyes peeled.

Hello Demi! Unfortunately you can change your gamertag only once for free! You would be required to get charged for the next gamertag change attempt, very sorry to hear you put the incorrect letters! Every gamertag change is $9.99 USD after the first one.

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https://support.microsoft.com/en-us/topic/change-your-gamertag-on-xbox-one-1bd92fa1-d3fc-7f0c-f95f-0d048fce6b40

That sucks, Alright I Will, Thanks

Some time ago, Microsoft changed it so multiple people can have the same gamertag...each separate one is identified by the numbers after the name. So, 2 people can have the gamertag "username" for example...one would be, say, "username4589" and the other "username3629". In the past it used to be only one gamertag instance at a time

Unfortunately, far as I know, there is no way to reverse it as you only get one free gamertag name change