You CAN switch methods (unless you lease) -- but you have to account for the "depreciation" component of the standard mileage deduction that you have taken and adjust the basis of the car for depreciation purposes if you switch to actual expenses, and there are restrictions on the depreciation method you can use. If you took actual expenses the first year, you can only switch to standard mileage IF you took only "straight line" depreciation when you were taking actual expenses.
From the IRS website: "Choosing the standard mileage rate. If you want to use the standard mileage rate for a car you own, you must choose to use it in the first year the car is available for use in your business. Then, in later years, you can choose to use either the standard mileage rate or actual expenses.
If you want to use the standard mileage rate for a car you lease, you must use it for the entire lease period. For leases that began on or before December 31, 1997, the standard mileage rate must be used for the entire portion of the lease period (including renewals) that is after 1997.
You must make the choice to use the standard mileage rate by the due date (including extensions) of your return. You cannot revoke the choice. However, in later years, you can switch from the standard mileage rate to the actual expenses method. If you change to the actual expenses method in a later year, but before your car is fully depreciated, you have to estimate the remaining useful life of the car and use straight line depreciation."
Also: "Standard mileage rate not allowed. You cannot use the standard mileage rate if you:
Use five or more cars at the same time (such as in fleet operations),
Claimed a depreciation deduction for the car using any method other than straight line, for example, MACRS (as discussed later under Depreciation Deduction), ***********note, you would only have take depreciation if you were using the "actual expenses" method ************
Claimed a section 179 deduction (discussed later) on the car,
Claimed the special depreciation allowance on the car,
Claimed actual car expenses after 1997 for a car you leased, "
Talk to someone who knows what they are talking about; there is a wealth of misinformation on that Tax thread in the new shopper area. Some of the information given early in the thread is changed by the author later on; if you don't read through the entire thread you won't see where that happens.
Tax laws change frequently; most of the information in that thread was not posted recently.
It's far better to talk to a professional or consult the IRS website directly. This is one of those "your mileage may vary" (no pun intended) situations. What is appropriate for one tax payer may NOT be the best approach for another. A person who is single, no children, and no other source of income than mystery shopping is NOT in the same situation as one who is married, has a spouse with income, has children, and who also has a part time job as an employee. The strategy for writing off business mileage to optimize the tax result may not be the same between two people if their situations differ. Even whether you own a home and are paying a mortgage may change the strategy regarding business deductions, home office deductions, and lease versus buy strategies. Your total income will affect how much depreciation you can write off.
Please don't take the one-size-fits-all advice given in this forum, except the advice that suggests you do your own research and talk to your own tax professional.
Time to build a bigger bridge.