Investors & landlords

So it's safe to assume the amount that turbo tax calculated for prior year depreciation was the amount eligible to be taken if I proceed without amending prior years or changing accounting method? If I do decide to file form 3115 do I have to file it again to change the accounting method back? The reason I'm asking is because I actually had 2 rentals, I sold one of them but I didn't depreciate on the other one either. I'll start depreciating on this current return unless the form 3115 always me to continuing doing it the way I have been.  And one last question, how is the depreciation calculated? What amounts did turbo tax use to calculate the eligible amount?

Thank you so much for your help.