You mean non-stop don't you? Direct flights go from point A to point B, but stop at point C.

As for changing, some of the fares are based on making a connection. So, it's possible that the fare you booked doesn't allow you to be on a non-stop or direct flight.

So, to try to change after ticketing, you'd be at the mercy of the gate or ticket counter agent.

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Not sure where I'm going, but I'm gettting there fast.