Let me help provide an option about removing usernames when logging into QuickBooks Online (QBO).
Right now, QBO doesn't have a feature to merge the two usernames. What you can do is, add a user and make it as the Master admin temporarily. (Use other email address)
Then, have the master admin remove your name on the list and re-invite yourself with the correct email. Once you receive the invite, you have an option to choose what user ID you want to use. (see screenshot below) Once everything is set up, you can now change back the Master admin to your name with the correct user ID and delete the user that you've added temporarily.