Thank you. Since it was impermissible method of depreciation for over two years, I understood it as a "adoption of an accounting method".
From Pub. 946:
How Do You Correct Depreciation Deductions?
If you deducted an incorrect amount of depreciation in any year, you may be able to make a correction by filing an amended return for that year. See Filing an Amended Return next. If you are not allowed to make the correction on an amended return, you may be able to change your accounting method to claim the correct amount of depreciation. See Changing Your Accounting Method , later.
Filing an Amended Return
You can file an amended return to correct the amount of depreciation claimed for any property in any of the following situations.
You claimed the incorrect amount because of a mathematical error made in any year.
You claimed the incorrect amount because of a posting error made in any year.
You have not adopted a method of accounting for property placed in service by you in tax years ending after December 29, 2003.
You claimed the incorrect amount on property placed in service by you in tax years ending before December 30, 2003.
Adoption of accounting method defined.
Generally, you adopt a method of accounting for depreciation by using a permissible method of determining depreciation when you file your first tax return, or by using the same impermissible method of determining depreciation in two or more consecutively filed tax returns.
For an exception to the 2-year rule, see sections 6.01(1)(b), 6.19(1)(b), and 6.21(3)(b) of Revenue Procedure 2019-43 on page 1107 of Internal Revenue Bulletin 2019-48, available at IRS.gov/irb/2019-48_IRB#REV-PROC-2019-43.
When to file.
If an amended return is allowed, you must file it by the later of the following.
3 years from the date you filed your original return for the year in which you did not deduct the correct amount. A return filed before an unextended due date is considered filed on that due date.
2 years from the time you paid your tax for that year.
