The stimulus is actually a 2020 stimulus based on the 2020 tax return. Prior checks were advances using prior tax return data. When you file your 2020 tax return there is a stimulus question that asks how much you have already received and if you are owed more based on the 2020 data, it will be added to your 2020 tax return.
what specifically do you want to amend on prior tax returns? it may not be necessary...
The way the stimulus REALLY works is EVERYTHING is based on you 2020 income, filing status and number of children under the age of 17 on your 2020 tax return. What these numbers were in 2018 and 2019 does not matter.
The IRS will calculate your stimulus based on 2020 numbers; that is what is considered the ACTUAL.
The two payments were just ESTIMATES; the 1st stimulus was an estimate based on either the 2018 or 2019 tax return, using whatever was available.. the 2nd stimulus was an estimate based on the 2019 tax return only
if the ACTUAL is greater than the ESTIMATE, you will receive that amount as a credit on your 2020 tax return.
if the ESTIMATE is greater than the ACTUAL, you are not required to return the money nor does it impact your refund. Line 30 on Form 1040 will be zero.
does that help?