What Julia and Khalid said. Yeah, you probably won't.
Yes, you can do it at the airport but they'll be very expensive. First of all, it's likely that you're purchasing a last-minute fare, which will be much more expensive than a flight purchased in advance. Secondly, some airlines even charge additional fees for booking in person or on the phone. Booking online is always the least expensive option.
I guess you can just call and talk to one of the flight attendants and they can reschedule your flight
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