The QB Admin people skirt around the issue and do not directly confirm what I told you. Look at the link below and PLEASE PLEASE PLEASE tell me this worked since I have NOT updated to Big Sur although I did update to QB2019 R10.
You should be able to run QB2019 and it should open that database. Your QB database should still be on your computer running Big Sur. If not, you should be able to log into your QB user account and download a copy of R10 and use your license number to activate it. Did you perform the QB update BEFORE upgrading to Big Sur? QB2019 versions below R10 will not run under Big Sur. I believe that if you had upgraded QB2019 to R10 that it would have run under Big Sur.