I understand why you're not amending the 2017 return. With a passing date in December of 2017, I'd probably not amend that one either. The "savings" would not be worth it.
I'm amending 2018 and 2019 returns this week to fix the errors.
Be aware of what that means, because you've got your work cut out for you. When you amend the 2018 tax return, that means your imported data in the 2019 tax return will be wrong. You can't import from a prior year, when you amend a return. So you'll have to manually figure the correct numbers for the 2019 return (namely prior year's depreciation and carry over losses if any) and change the numbers that were originally imported from the 2017 return. You'll have to do exactly the same for the 2018 return since it's originally imported numbers will be even further off.
Once you get the 2019 return amended, then you can start your 2020 tax return over from scratch and make sure you import from the amended 2019 return, and not from the original. (This matters big time if you originally filed the 2018 and/or 2019 returns using the online version of the program.)
As a reminder, a prior year tax return or prior year amended tax return can not be e-filed. (The IRS says so). You have to print, sign and mail the returns. If a state return is involved, that gets amended too and mailed to a physically separate address from the federal return. Additionally, each tax year gets mailed in a physically separate envelope.
Make sure you write down the 2019 AGI from the "originally" filed 2019 tax return. You will need that AGI in order to e-file your 2020 tax return. Since amended returns are processed by hand it can take anywhere from 12-16 weeks for it to be processed. So there's no way the "new" AGI on the amended 2019 tax return will be processed before you e-file the 2020 tax return. Therefore, you'll have to use the original 2019 AGI in order to e-file the 2020 tax return.
I also recommend you mail the 2018 amended return first. Then wait about a week to mail the amended 2019 tax return. Otherwise, if the IRS starts processing your amended 2019 tax return before the 2018 return, they "will" find what will be interpreted as major problems which will result in an "audit by mail". You've got enough headaches to deal with now. So don't be in such a hurry to create even more.
Finally, I do recall having read in some IRS Pub that for situations like yours, when you've not depreciated for 2 or more years, or have incorrectly depreciated for 2 or more years, amending isn't an option and the 3115 is required. For the life of me I can't find the IRS pub that says that. So either my memory on what I read is skewed (very possible) or I"m just not looking in the right place. This is just a "heads up" in case you get feedback about this from the IRS with your amended returns.
