Yes, you can switch methods, but you cannot do so without changing your accounting method with the IRS. To change your accounting method, you'll need to file form 3115. Form 3115 is due by the due date of the return that the accounting method change is being made for, including the extension.
If you'd like to prepare a Form 3115, you'll need to use a Desktop version of TurboTax, and file your return by mail.
See: Application for Change in Accounting Method