During registration on this forum, I right-clicked the element that showed Username, removed disabled=“”, and changed it to my current username: Eoan. (This element normally cannot be edited, as it is locked to Gamertag when signing in from Xbox, or presumably might match Steam account if signing in with Steam.)
I’m testing to confirm that this worked. This is a second account. On my first account, this seemed to have worked, but I logged out (deleted cookies) to try to change display picture, and when I logged back in again my username (and the url of my profile here) matched my Gamertag.
So this account’s username might later change to my Gamertag, which is ForgotGamertag9.
Notable because Eoan was not available as a Gamertag, so if that user were to try to create an account on these forums at this moment, they might get an error or something.