I have successfully done this twice now on NWA. The "premium" seats that you can pay for become available 24 hours before your flight. On NWA, these seats are generally an additional $20-$25. On both occasions, exactly 24 hours prior to a flight, I have upgraded to these seats at no charge. It seems that at that moment, the seats show up with a $0 upgrade fee. Changing to them produced an error on the web page, but the change goes through at no cost. Subsequent views of that page show that there is an upgrade fee now associated with those seats.
You can do that with Continental also. You can check in online within 24 ours of your flight and all seats are available to choose from.
The point of my post is that on NWA, the premium seats to choose from cost money. Twice, while changing exactly 24 hours prior to my flight, I was able to change at no cost as the cost on the seat showed $0. 23 hours and 59 minutes prior to the flight, the cost on the seat showed $20 or $25. 24+ hours prior to the flight, the seats were unavailable.
