@Anonymous
Since there are no tax changes for 2018 and 2019, I suggest you update the basis on the current return to the correct basis. Make the adjustment to the basis and follow through and make a note for your 2021 tax return, for your information, how you re-calculated the basis should there be questions later. IRS is interested in correct taxes based on correct income information. You do not have to file an amended return, but make the corrections and be able to show how you calculated the adjustments if IRS ever ask for documentation of changes.
Write the letter, that you speak about, to the IRS as a explanation for the changes and keep it in your files.
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