This is tricky - I have no experience on this whatsoever.
However, if you had a confirmed TICKET (not just the upgrade pdf) , i.e if the terms and conditions of your carriage changed to show you that you are in C-class then you may have some luck under EU261. However I think you may just have a pdf confirmation, which wouldn't constitute a ticket. But dont expect A3 to budge that easily, and you may have to go to the regulator.

However, if I may add my (unsolicited) opinion: You incurred no financial burden, and because the nature of a Y->C paid upgrade , the upgrade itself wasn't guaranteed. So, you literally got what you "paid" (in miles) for and the only damage you got is A3 holding your money for ~36hours, plus the cost of a phone call. It would be really hard to argue to anyone that you are owed compensation because you incurred some form of damage