If the flight doesn't exist, Delta will eventually catch up and move. Alternately, you could call back in and tell the rep that your flight doesn't seem to exist any more and you'd like to change to any other valid routing. You've got months until your flight and Delta will take care of you in this situation. Don't worry. Either let it ride until Delta's algorithm catches the canceled flight, or call in and see if you get a better phone agent who sees that the flight has changed.