So here is how the story ends. Aegean did call back as promised. I agreed to change the flight to the next day. For the Euro 250 I had to email my bank information. They acknowledged the email with another phone call. The money arrived within a day. And for the free flight within Greece I have to call them and mention the rebooking within one year.

I think they also said that if I cancel the rebooked flight I will get Euro 49 back of the original purchase price.