I had asked for an upgrade a week before. I called in in the morning, but was told to ask at check-in again. Eventually, I was upgraded 20 minutes before departure and boarded last. The crew was confused, as I wasn't entered into their iPad App. Both gate agents verified it. The FA then informed me that there would be no c-class food. Eventually, she found some and served it anyway. My domestic connecting flight was also upgraded during my international flight and I got the new ticket at the gate at ATH.
In the end, I was credited c-class miles for both flights, as has happend the years before with super late upgrades. I thought that was quite generous. It is very difficult to make arrangements for luggage with these super late upgrades, however. My return flight has already been upgraded instantly, while the domestic sector is still uncertain. Both flights had more than enough c-class space and both flights weren't full. Good luck to you!