I've found that if they swap the aircraft type, seat assignments are disabled until check-in.

I'm on a flight tomorrow that was originally scheduled with an E190. When they upgauged it to an A320, seat assignment was disabled. When I checked in, it allowed me to check the seat map. It looks like anyone who had a window seat was moved to a window seat in the same row, aisle seats were moved to the aisle seats, and EMS exit seats were moved to the first exit row. There were only middle and EMS seats left. Since there were more EMS seats, the price for EMS dropped. I was able to snag a window seat in the second exit row for a decent price.