Lets make sure you're doing this right. I doubt it, since your error goes all the way back to 2010. Most likely, you just flat out may not have the correct amount of prior year's depreciation already taken. But if you want to risk making a big mess even bigger, that's your decision. In the end, it may come back to bite you in a way that will make the cost of professional help seem like an absolute pittance in comparison.
First, you enter the 2017 tax data into TurboTax 2017 exactly as originally filed. Then you select the "print and file" option and actually print the return as originally filed. This tells the program that you have "in fact" filed the return. Compare the printout to the return you actually filed. It should match exactly. Save the .tax2017 file at this point and completely exit the program. You will "NOT" amend the return at this time.
Now fire up TurboTax 2018 and the first thing you'll do is import the data from the .tax2017 file. This ensures your 2018 tax return will match exactly the one you actually filed. Then complete the 2018 return exactly as originally filed. Select the "Print and File" option and actually print the return. Again, this tells the program that you have in fact, filed the return. Compare the printed return to the one you actually filed. It should match exactly. Now save the .tax2018 file and exit the TurboTax 2018 program. You will NOT amend the 2018 tax return at this time.
Now fire up TurboTax 2019 and import from the .tax2018 file. As above, this will ensure the imported data will match exactly the originally filed return. Complete the 2019 tax return exactly as filed, then select the "Print and File" option and print it out. This tells the program that you have in fact, filed the return. Completely exit the TurboTax 2019 program. Compare the printout to the 2019 return actually filed. They should match exactly.
Assuming all is good at this point, you're ready to start amending.
Now fire up TurboTax 2017 anew and before you open the actual return, click the Amend button. Then go through the process to amend. You'll just work through the SCH E section as you did before, making your necessary corrections. Once done, you have no choice but to "print and file" the amended return. The program will only print those forms you actually need to mail to the IRS, along with mailing instructions. You will mail this 2017 amended return in it's own separate envelope from other amended returns. This will complete the amending process for the 2017 return.
Now it gets tricky, so attention to detail is important.
Fire up TurboTax 2018, and before you open the .tax2018 file click the AMEND button. Take note that when amending a tax return, you can not import from the prior year's tax return. Therefore, the "prior years depreciation" amounts are going to be WRONG. (I guarantee it, since you changed things to 100% business use on the 2017 tax return.) So as you are working through the SCH E section of the 2018 return, you will manually change the amount in the "prior year's depreciation" box, when presented that screen.
Take special note on just how to correctly figure the prior year's depreciation here, as you will be required to do some manual math on your part.
If you'll take a look at the 2017 IRS Form 4562 that prints in landscape format, titled "Depreciation and Amortization Report" you have to add together the amounts in the "Prior years depr" column and the "current year depr" column on the 2017 form 4562, in order to get the correct total of prior years depreciation to enter on the 2018 tax return.
