You didn't say which returns, so I'll assume you mean prior returns. Have you located the prior returns, and are they in the SAME account as your 2018 return? When you try to download/print, does it say you need to pay to access?
And so you upgraded your 2018 return to PLUS, but how did you pay? Did you pay the 19.99 by credit/debit card? You cannot use the method to pay out of the Federal refund for this purpose. So if you paid by credit/debit card, any prior returns in the same account should no longer be locked.
Have you already filed the 2018 return, or is it still in preparation? We can tell you more when you answer all the above questions.
