Originally Posted by RubberCityRebel
Is there any way to use miles to upgrade an already purchased T class ticket?
Welcome to FT. You will find lots of wisdom and a lot of other things on the site.
To answer your question, Delta's website tells us that it would cost you 10,000 miles to upgrade but it depends on if there are any seats available to be upgraded.
Within or between the continental US, Alaska and Canada
One-way, One-Class Upgrade From Select Fares1 10,000 (D511)
Your best bet is to call DL and find out if there are any seats.
For future reference, just click on the SKYMILES tab on the delta.com website and it usually will answer your questions.