Don't know if this is something new but just looked at the upgrade voucher T&C:
"The member is required to have a valid and paid Economy Class ticket with confirmed reservation either with Aegean as the validating carrier (stock number 390) or with Olympic Air as the validating carrier (stock number 050)."
I thought that it was before only with A3 issued tickets, but now seems to be also for OA. Or maybe I remember it wrongly..