You can't downgrade since you've already filed. If you are in Deluxe and try to pay now, it will charge you 59.99 for Federal, and possibly 44.99 for a state return.
I'll tell you in an Answer below how to phone TurboTax Support and tell them you upgraded too far by accident. If you are lucky, they may just unlock the 2017 return for you.
Another option is a free 2017 tax return transcript from the IRS. You may be able to immediately download a 2017 return transcript at the IRS website at the link below. If you don't qualify to open a free IRS account there, you can order one to come by postal mail, which normally takes 5-10 calendar days.
<a rel="nofollow" target="_blank" href="
http://www.irs.gov/Individuals/Get-Transcript">http://www.irs.gov/Individuals/Get-Transcript</a>
