Switching from Actual Expenses to Standard Mileage

What depreciation method has been used?

A new vehicle would have to be purchased to switch back to mileage.

Generally, the Modified Accelerated Cost Recovery System (MACRS) is the only depreciation method that can be used by car owners to depreciate any car placed in service after 1986. However, if you used the standard mileage rate in the year you place the car in service and change to the actual expense method in a later year and before your car is fully depreciated, you must use straight-line depreciation over the estimated remaining useful life of the car. There are limits on how much depreciation you can deduct. For additional information on the depreciation limits, please refer to Topic no. 704. Publication 463 explains the depreciation limits and discusses special rules applicable to leased cars.

If you have fully depreciated the car you can only use expenses, but no further depreciation 

Yes, you can switch back to the Standard Mileage Rate.  I am positive.

As a side note, I almost guarantee that the depreciation for Actual Expenses was done incorrectly.  I'm not aware of ANYBODY (including me) that does it correctly.  🤣