I assume your 2017 and 2018 returns are in the same online account.
If you have already filed your 2018 return in Free Edition, and are now trying to upgrade the filed 2018 return to PLUS in order to unlock a 2017 return, and are now in Deluxe, then you have upgraded too far. If you were to try to pay now it will charge you for Deluxe and possible a state return.
Since you have already filed the 2018 return, you won't be able to clear it to downgrade it yourself. You'll need to phone TurboTax Support on Monday, tell them you upgraded too far after filing, and ask them to unlock the 2017 return for you. They may not even charge you.
Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday.
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
There's also a free option if a 2017 return transcript will suit your purpose. You may be able to immediately download one at the IRS website if you qualify to open an IRS account there. Otherwise, to order a transcript to come by postal mail takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript
