If I upgrade to Premier to file 2018 taxes, can I then access my 2017 taxes (done under the free version) to file a 2017 amended return?

OK ... upgrading  the 2018 program will not help in your situation but you do need to pay for the 2018 program to unlock the prior year return  ... what would be best is to return to the tax place and have them amend the return.  

If you insist on using the TT program it will take a few steps and you will need to buy the 2017 program first... and start a dummy return  ... then download the 2017 tax file ... open it, complete the FILE tab to choose to mail the return ... close the program ... restart the program and  then start the amendment process. 

How do I finish a prior-year return that I started in TurboTax Online?

If you started your 2015, 2016, or 2017 return in TurboTax Online but didn't finish it before the shutdown, you can always start over in the TurboTax CD/download software, available for purchase at TurboTax.com.

You can also transfer your incomplete return to the CD/download software if you don't want to start from scratch. Here's how:

1.    Purchase the TurboTax CD/Download software and install it on your PC or Mac computer.

o    If you're also doing a state return, be sure to download and install the corresponding state program on your PC or Mac.

2.    Now sign in at TurboTax.com with the same login you used to prepare your prior-year return.

3.    Under Your tax returns & documents, select Download .tax file (not the PDF) for the prior year you want and save it to your computer.

4.    Back in the CD/download software, select Find a Tax File, and then browse to the tax data file you downloaded in Step 3. Select the file and click Open.

5.    You can now finish up in the software. You'll need to paper-file, as e-filing for prior years is no longer available.

Related Information:

·         How do I get a copy of a return I filed this year in TurboTax Online?

·         What's the difference between the tax data file and the PDF file?

·         Can I convert my PDF into a tax data file, or vice-versa?

If you did not pay the 2017 TurboTax online account fees, you will need to purchase and download the 2017 desktop editions to complete your return.7

If you did pay the 2017 TurboTax online account fees, then you will need to contact TurboTax support to have them push a copy of the desktop edition to your download account.

Use the following TurboTax website to contact TurboTax customer service/support.  Support is open from 5am to 5pm Pacific time (8am to 8pm Eastern time) M - F.  Submit your question/problem during business hours and then use the options for either Chat or Call (a phone number will be provided for Call) to speak directly with a TurboTax agent -

https://support.turbotax.intuit.com/contact/