Oh, it's an A3 flight! Well, A3
states in its site:
Upgrade Award can be applied only on Aegean operated flights (both domestic and international). You are required to have a valid and paid Economy Class ticket with confirmed reservation to get yourself upgraded
(highlighted by red coloured font by me)
In other words, award tickets can't be upgraded. Perhaps a stupid rule but, still, published by A3, and easily found on their site with one single, simple google search, though. The old OA had no upgrade possibilities, no C on domestic flights and no real internet pages... 