It depends if you are Gold Card Member or Silver Card Member. If you have a gold card, sometimes if the flight is full, they automatically upgrade you to the next level. I am a silver card member and have been upgrade couple of times when the flight is full.

But thats a chance you take, in case the flight is not full, they dont upgrade, you could request but its unlikely.

Even if you pay at the airport, the price would be the same as paying before.