To catch up on missed depreciation, you must make a change in accounting method. However, this is an automatic change; you don’t need IRS approval but do have to follow procedures. Details on how to do this can be found in Rev. Proc. 96-31 (https://www.irs.gov/pub/irs-tege/n96-30.pdf).
Items directly reducing income. Personal deductions such as for mortgage interest, state and local taxes, and charitable contributions are allowed only if deductions are itemized on Schedule A, but deductions such as for alimony, capital losses, moving expenses to a new job location, business losses, student loan interest, and IRA and Keogh deductions are deducted from gross income even if itemized deductions are not claimed.