This happens almost every time I have a schedule change on one of my reservations. The 100% full seatmap that is shown is a glitch and doesn't reflect the real seatmap. The problem might correct itself in a few days, or you can just call AA, let them know you've had a schedule change, and ask them to re-synch your ticket. Once that's done the accurate seatmap will reappear and you'll be able to select seats.
ETA: The seatmap for flight 2956 on Oct 15 is pretty wide open right now.