No, you don't need to amend your pervious years returns. You can take mileage deduction in 2020, as you did in pervious year. Based on your description you didn't take Section 179 depreciation. If you plan to use your vehicle for personal use only starting this year, your only concern should be the cost basis of your car in case if you decide to sell it. Since you took MACRS depreciation deduction, your cost basis was decreased, and you might be a subject a tax on gain on sale.
