Sorry, but you do have to start all over, and import anew, right at the start of the new file.
IF the 2020 change was minor enough, sometimes it's possible to adjust the partially-prepared 2021 one for whatever was change on the 2020 file, but that depends on what was changed.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
