East_and_West

Then this should doable, based on fare availability of course. (Since it was not clarified in your post, I just assumed that you were flying ATH-OTP-KRK-OTP-ATH, so I ruled out the OTP-KRK segment being operated by A3...) Note though that since the recent changes, the number of seats given in advance through the upgrade vouchers has been reduced. But even if there is no I class availability in advance, you may get upgraded at a later stage, even at check-in. Then since your bags are checked all the way through this shouldn't be a problem. But better give them a call to make sure, and avoid any unpleasant surprises at the airport.