With rental property, if you did not depreciate, or if you used an improper method for "2 or more consecutive years", then amending is not an option. You have to file IRS Form 3115 with your current tax return in order to fix this.
IRS Publication 946, https://www.irs.gov/pub/irs-pdf/p946.pdf page 13:
The following are examples of a change in method of
accounting for depreciation.
• A change from an impermissible method of determin-
ing depreciation for depreciable property if the imper-
missible method was used in two or more consecu-
tively filed tax returns.
• A change in the treatment of an asset from nondepre-
ciable to depreciable or vice versa
