On Delta you need to "accept" the changes to your flights (over the phone, or if you insist, by mail or email) before you can get new seat assignments confirmed.

Actually the act of calling on the phone to request the same or different seat assignments will complete the acceptance of the changes to your flights.

Other passengers use this opportunity to request different flights that fit their schedule better than the automatically rescheduled flights.

Travel tips:
http://members.aol.com/ajaynejr/travel.htm

It is a good idea to "accept" new flights as quickly as possible. I think that once a flight was cancelled out from under me and I was automatically "protected on" (rescheduled to the nearest) a different flight (it was Delta). I saw the rescheduling had happened, left it alone for a week and then found I was rescheduled a second time to another different flight I liked even less. I called and managed to get back on the first re-scheduling (it was sold out) after a brief argument "that I made this reservation way way in advance" but next time I will be more vigilant and "accept" flights more quickly. Seems like the unaccepted reservations were re-rescheduled shortly thereafter for load balancing purposes. Knowing if and when flight changes occur may be difficult but in this instance I was forewarned because it was in the news that a lot of flights were being cancelled, it was ca. 9/11/01. Starting the conversation with simply requesting seat assignments seems like the most innocent way of accepting the flight changes at least in the instance I described, where this will be least likely to call attention to the sold out status of the flight, and Delta is now one of those airlines where passengers with seat assignments do have a slightly less of a chance of being bumped or even re-rescheduled.