For me, upgrade worked once and whenever I restarted the machine after first successful login (immediately after upgrade completes), it used to not take correct password for any of the preexisting accounts. This is a bug in Big Sur.
What I did was - I created the new admin user account
right after the first login after upgrading to Big Sur. Somehow all preexisting account started working as well. You can later on delete the newly created user account if you wish. I am enjoying the Big Sur new look after a day of hard work. I had to figure this out in a very difficult way by installing Catalina multiple times and going through many post online ( none of them work for me but gave me an idea to create new user after first login). Hope this helps.
Hope this helps.