Hey @TomMurphyGDCL,
It has been awhile since I've dealt with this, and I am no longer in support but providing some direction which I have previously used to help people out of similar binds in the past, as you're in the wrong forum for this issue. Next time reach out to the support org via the regular channels (submitting a ticket via the accounts portal at manage.autodesk.com) to get responses. So the previous advice that's helped others is as follows:
Autodesk cannot make changes to a user account as requested as single log-in cannot be two different accounts for security purposes, and it is not as simple as a name change - the previous account has to remain for both data integrity and procurement reasons (someone paid for access one way, and no longer is, and someone is paying for access in another way now) so we can't 'merge' the account. This is the sort of thing which users need to address before the update - NEVER CLOSE A MODEL WITHOUT SYNCHING YOUR WORK OR RELINQUISHING ALL ELEMENTS.
While we cannot 'revert' the username, you can manage to get out of the bind by following the steps here: https://help.autodesk.com/view/RVT/2023/ENU/?guid=GUID-A65C9B9D-CB0B-4678-B02B-80BA8164B38F
This will need to be done by a user with project admin rights (likely the BIM manager for the project, but it might work for you as well). Note that if you're in Revit 2021, 2022, or 2024 you will need to change the address (the 2023 changes to the year you need to see).
I know it stinks to have to ask someone else to clear this up for you, but we do not have access as a rule. As a result your project data is yours to work with and edit, not ours.
Hope this helps,
-jacob