It totally depends on your class of service you are booked in... Averything in Q, S, G,W,U,T etc. cannot be upgraded at the check in. Y,O, B... classes can be upgraded at the airport against a lot of money. If you have a miles and more account with enough miles you maybe able to use your miles for an upgrade. Other than that you can try to get the seats "NewbieRunner" was talking about. But without the M&M status it's going to be tricky...