Help @MojangSupport. I was trying to migrate my account and I used somebody else's saved account details. As soon I put it in I closed the tab and went to try again having not finished the migration steps. I then open it in a new tab and I'm still signed in to the wrong account.
Please send us an email at https://msft.it/6014XDReu and we will assist. Remember to attach your Transaction ID and account name / Gamertag.