I converted my primary residence to a rental in 2010. I have been using $150k as basis (purchase price) for depreciation, but current research says I should have used $120k (fair market value at the time of conversion).

It looks like I cannot amend tax returns more than three years old.

Should I continue to use the $150k basis, or correct the last three years to use the $120k basis, or is there another option?

Thank you!