Originally Posted by jjl468

Your understanding is mostly correct, but there are a few more restrictions to be aware of. The ones for Economy are that your original economy ticket must have been in a higher fare bucket and that there must be OY (i.e. GUC) upgrade availability. If you can find the receipt for your original flight, you can check if your fare bucket is eligible, with H/Q/K fares being eligible for a 65,000 mile upgrade and Y/B/M fares being eligible for a 35,000 mile upgrade.

Delta has the fare class restriction in place so that it doesn't bleed money by offering this, ensuring that part of the money you save by following this path is made up for by an expensive Economy ticket (or just a Premium Economy ticket). Since you booked Economy originally and not C+, I would expect your fare class to not be eligible as most people would actively avoid these higher fare classes. You can double check by looking at the receipt for your original booking, which will tell you what your fare bucket was.