Hmm that is a good point. I guess I assumed this to be a rule as it pretty much always seems to be the case.
I called Aegean again: No can do with 050 ticket. So for future reference: Be VERY careful when booking through an OTA: Even with Aegean flights it may be issued through Olympic and thus not upgradable.
Turns out that also availability has either now dried up, or I checked incorrectly before. I could have sworn it showed as available on the united website yesterday. Now not any more according to the Aegean lady, so I guess not much use in changing the flight selection to get a 390 e-ticket.. (if that would even work)
Seems I wont be able to use my coupons before November then. Thats really too bad!