Firstly I'd like to thank Krishna Prasobh V for directing me to the correct place to post this.
Original: http://answers.microsoft.com/en-us/windows/forum/windows_10-other_settings/cant-change-xbox-gamertag-once-for-free/d84aa864-dfc7-4c66-bce4-08bb893404a1?msgId=9c84d91a-2acb-4c4a-a874-97d0fd7316ff
I'm having trouble with changing my gamertag; the problem started when I made a suggestion on the Minecraft Windows 10 BETA site and realised that it was using my "old" username, despite the only name I've ever associated Minecraft with is my new one, I asked support if they could change it and they directed me to the Xbox gametag page telling me it can be changed once for free, this was strengthened as it said the same thing on the page; after accidentally updating my security info thinking I was doing something else and having to wait a month, I was finally able to change my gamertag.
Now I tried changing it and it's asking me for my credit card details saying it'll be charged immediately with no refunds, but I've never changed my username before, this should be the first "free" time.
Any ideas?
(Changing it via PC as I don't have a console anymore)