Since you never did include the improvements for depreciation, you have two choices. You can omit them or you can use form 3115 to include them. TurboTax does not support this form.
Below are the IRS links related to the change in accounting method. TurboTax does not have that form.
May be these will help
Form 3115, Application for Change in Accounting Method
Form 3115
Instructions for Form 3115 (03/2012)
Instruction for 3115
Form 3115,
Form 3115 pdf
