Agree with Mr. Smiley Guy that your client missed the boat for amending prior year returns.

Rev. Proc. 2018-31 Section 6.07 does provide relief and considers the year of disposition as the "year of change", where the change is from an impermissible method of accounting for depreciation to a permissible method of accounting for depreciation (which includes a change from treating an asset as nondepreciable to depreciable in accordance with the tax law).

Since depreciation allowable must be taken into account in the determination of gain regardless, the filing of F.3115 will not directly change the amount of gain that needs to be recognized but it could because a §481 adjustment will be allowed (without the need to adjust any suspended PAL).

I would have to disagree with Bill though, that §121 exclusion CAN be applied against depreciation that was never actually taken on the basis §121(d)(6) refers to §1250(b)(3), which has the usual allowed or allowable provision.