Yes, you can change the basis on your 2018 return.You can also go back and correct the basis on your open tax returns (2015, 2016, 2017) by amending your tax returns for those years, using the same 27.5-year schedule. But you can't go all the way back to 2004. Using a wrong Basis for depreciation can only be corrected by amending your tax returns, and you can only amend up to the 3 prior years. Also, the due date for amending 2015 is 04/15/2019, unless you filed your 2015 later with an extension.
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