A Greek friend succesfully upgraded my OTP-ATH-BRU flights without problem, using her vouchers she couldnt use herself.
Only slightly weird aspect: the A3 agent said there was availability, but couldn't upgrade instantly as HQ had to give a green light, so they would call back. But when my friend informed me about this right after the call, and I checked right away on Aegean manage booking and checkmytrip.com, it already listed the flights as I class and business class on both websites respectively.