The Roth conversions after 2009 included a pro rata share of basis even though it was not reported properly on your tax return. Failure to report it properly on Form 8606 does preserve that portion of your basis for future use. You'll need to reconstruct the the previously filed Forms 8606 to correct the calculations to determine the amount of basis that carried forward each year. For 2021, 2022 and 2023 Forms 8606, you can file an amended tax return and claim a refund, but you have to do the one for 2021 by April 15, 2025 before 2021 becomes a closed tax year. You'll need to include an explanation with the first of these amendments that you file since this will need to appear as an adjustment to your basis. It seems unlikely that the IRS will ask for the intervening corrected Forms 8606 because they are not reporting any additions to your basis, but they still might ask for them. (If you had no reason to file Forms 8606 in 2021 through 2023, you would make the adjustment on your 2024 tax return and provide explanation.) An adjustment is made in TurboTax by clicking the EasyGuide button that TurboTax presents after you indicate that you made nondeductible contributions.
