If your 2018 return is now in Deluxe, then you have upgraded too far. You cannot downgrade a return that has already been filed. If you try to pay now, it would charge you for Deluxe Federal and possibly a state return, if applicable.
Since you are in Deluxe, you should have phone privilege. Here's how to reach Customer Support so you can tell them you upgraded too far accidentally and ask them if they will unlock the 2017 return.
In this tool, where it asks what product, choose "Online", and then for Which Edition, choose "Deluxe."
In the issue line say (without quotes) "Please unlock 2017 return. I upgraded too far by accident."
https://support.turbotax.intuit.com/contact
Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Mon-Fri
