1 and 2. No, you are not permitted to simply update your basis when filing your 2024 tax return. Filing Forms 8606 to report nondeductible traditional IRA contributions made for the year are essentially required by law. You must file the particular year's Form 8606 for any year for which you made a nondeductible traditional IRA contribution. It's not clear whether you made any such contributions for 2020 through 2022. If not, you just need to file the 2019 Form 8606 (along with Form 1040-X) and a corrected 2023 Form 8606 (along with Form 1040-X). The Forms 1040-X will only show that there were no other changes to your filed tax returns and to provide a place for your explanation that you are filing the missing or corrected Form 8606.
