I don't understand the problem. When I go to the website "setup.office.com". I have to log in with the same Microsoft account I initially used when I first bought Office 2019. In that sense it is still connected to my Microsoft username and password.
Note that to log into my Microsoft account I go to the website:
Sign In with your Microsoft account. One account. One place to manage it all. Welcome to your account dashboard.
account.microsoft.com
After I log in with my Microsoft username and password the only product I see is Office 2021.