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).
A 12-month period ending on the last day of any month other than December. Partnerships, S corporations, and personal service corporations are limited in their choice of fiscal years and face special restrictions.