Then you are required to recapture any "accelerated" depreciation (which includes the Special Depreciation allowance.

Essentially, you need to recalculate things using the Straight-Line method (and no Special Depreciation Allowance) for all previous years, then enter that as a "recapture" on the bottom of the 4797 (I use the "enterable" Form 4797 to manually enter it).

https://www.irs.gov/publications/p946#en_US_2018_publink1000107689