In addition to ammending the 2020 form 1120-S, you may also have to issue corrected K-1's, and the recipient's of those K-1's may need to also amend their 2020 personal 1040 tax returns. As for amending the personal tax returns, it only needs to be done if the corrected K-1's will make a difference to their tax liability, or change any NOL's or carry over's.
