s there any way to recoup my lost deprecation above and beyond only amending the past three years returns?
Actually, to correct this you really don't have any choices since it's been done incorrectly for the last ten years. You need to file IRS Form 3115 - Change in Accounting Method with your 2020 tax return in order to fix this. While the TTX program does include the 3115, it's not simple by any stretch of the imagination. Additionally, the program provides practically no help or guidance for the 3115 either. You will need to seek professional help to ensure this is done correctly. Completing the 3115 incorrectly has the potential to be more costly in the long run. On top of that, if your state also taxes personal income, this could be a double-whammy. Please seek professional help to get this fixed and get everything back on track.
