One was placed in service in 2006 (should be 40 years)
Correct.
and the other in 2020 (should be 30 years).
The use of form 3115 is not a recommendation. It is a requirement. You have been using what is referred to by the IRS as an impermissible method of depreciation. Per IRS Publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf beginning at the bottom of page 13:
Generally, you must get IRS approval to change your method of accounting. You must generally file Form 3115, Application for Change in Accounting Method, to request a change in your method of accounting for depreciation. The following are examples of a change in method of accounting for depreciation.
• A change from an impermissible method of determining depreciation for depreciable property if the impermissible method was used in two or more consecutively filed tax returns.
Now while TurboTax does include the 3115, it is not simple by any stretch and the program provides no help or guidance in correctly completing the form due it it's complexity depending on the given situation. Therefore you should seek professional help with this. This is especially true if your state taxes personal income, as doing this form wrong can (and usually will) end up costing your more that the professional help, in the long run.
