OP, I've had the same over the past few weeks. About 2-3 weeks ago I woke up on Saturday to see a flight schedule change. Couldn't do anything with the reservation in "My Trips". Did a dummy booking and could see there was a flight at the same time with a new flight number. Seat map showed the seats I had as occupied so I didn't worry too much. Took until Sunday morning before everything cleared up and I had the same seats and flight time. Only change was the flight # change (DL Connection flight, same regional operator).

Same thing happened this weekend though at first I was more nervous as when I tried a dummy booking, there was no flight showing at that time (~1:00 PM), only a 6:30 PM flight which wasn't going to work connecting to my international connection. A little while later though my 1:00 flight was showing as bookable again, just under a new flight number, again (same DL Connection Carrier).