I think this should work.

sp_change_users_login 'dbo', 'dbo', 'Update_one'

The issue at least to me - was my dbo was mapped to the wrong user. And that is a no-go.

You will get this error:

"Terminating this procedure. 'dbo' is a forbidden value for the login name parameter in this procedure."

In my case, I checked the user properties, and the bad map was gone. I was able to continue after this fix and drop the login.