Originally Posted by SeattleDavid

This problem would occur if you waited until the gate to try and change your flights - but check-in agents are empowered to make these changes and then you get your new flights before you check any bags.

The only disadvantage is that you cannot do this until you arrive at the airport, which makes it a more painful process if the flight you want is 3-4 hours earlier. In my case my first flight was the same, it was only subsequent flights that I was changing.