Wrong Basis for Rental Property and 3115 form

I have a client that had multiple tax preparers in the past, the client had claimed depreciation on the rental property but later stopped claiming the depreciation. Now looking at the past depreciation take it shows that the depreciation was lower than it should have been. I am stuck between leaving the depreciation as it was initially elected or making adjustments on the 3115 form, along with claiming the missed depreciation. I have read that "mathematical or posting errors " are not able to be changed on the 3115 form. Then again I have seen that depreciation can be claimed if less was taken than the allowed amount. If someone could give me some advice, guidance? Thank you

I seem to think that it would be a change of accounting method - incorrect method of no depreciation changed to macrs.

I expect others to disagree, but I don't see it as an accounting method change. I see it as just a mistake.

Use the original depreciation method and the TP can file 1040Xs for the open years. Keep in mind that basis is reduced for allowable depreciation not deducted.