Once the return of capital or nontaxable distributions have been received up to the amount of your original cost, all amounts in excess of that basis become fully taxable. It is up to each investor/taxpayer to track this and then begin to report taxable income from these nontaxable/return of capital dividend distributions (receipts even if reinvested) when the basis is zero from recapturing this income tax-free. It's not optional when to report on your tax returns.
If you reported these dividends as taxable when you were not required to, you can amend your returns now as far back as 2017. The 2017 tax return must be filed no later than April 15, 2021. After that date you can no longer receive any type of refund because the statute of limitations will expire. You have some time to amend 2018 and 2019.
[Edited: 03/07/2021 | 11:55a PST]
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