KLM Offers ATH-AMS-EIN that perhaps could work for J class, at least theoretically. The last segment operated by bus - though having a KL flight number.

The problem is that Aegean does not like to rebook out of its own flights and definitely not out of the alliance. So even though they could rebook onto Transavia or KLM, I'm not sure if they are willing to do that. Likely it will be a fight, which is not helped by the fact that most people believe that 261/2004 only applies to the two weeks before departure. Airlines cleverly use that misunderstanding to get the pax to accept a refund rather than rebooking.

Let's say that they do allow you to change from the EIN to AMS flight. Towards EIN there is plenty of upgrade availability, to AMS there is almost none. Would they honor an upgraded ticket in such case, if the new flight has no I-space? I'm guessing they probably won't. In fact I would not be surprised if that would not allow you to get the upgrade coupon refunded either.