I think its how you word it and how the agents are/aren't explaining it.
Can you get a full refund of the upgrade......probably no unless it was a flexible fare which is unlikely.
Can you change back to economy if the fare is changeable still.....yes.
However would you want to do this? If the fare is changeable you would be paying any change fee but the fare is most likely non refundable so you wont get the fare difference back
You will get the difference in tax/YQ back but it probably still works out at a net cost rather than a refund.
Unfortunately i would say chalk this one up to experience and leave as it is!