In my experience Aegean doesn't allow any seat choice at online or kiosk check-in at all f the passenger has chosen a seat at some point before. And neither allows any changes to the seat choice after check-in. Which is very inconvinient and irritates me most of all in Aegean experience. Just because of that I always delay my check-in as long as possible. And before doing it I go check available seats at "my booking", where it's still possible to change seats (before check-in).