This isn't a simple "taking away" schedule changes and equipment changes do happen all the time. Those things do impact the inventory available for upgrades, that makes sense but it is a bummer when it's your itinerary that is impacted. As meducate said, it's important to keep track of your itineraries, seats and schedules via My Itineraries up until you're on the plane. You found out the reason why and were able to make changes so you didn't have any unwelcome surprises on the day of travel (i hope).

Enjoy your trip.