I've noticed similar happenings over the past year or so. I have yet to actually have a schedule change that severely impacts my travel days or times, but I have had a handful of unfortunate downgauges out of SLC from 757s to A320s
which crushed any hope of upgrades, and led to a scramble to make sure I still had good seats. If I'm lucky, DL calls me with a vague schedule change recording, I notice the departure is a little different, and the seatmap is the same, and since I was notified immediately (or at least as soon as the system decided to notify anyone), I could still get a good bulkhead or other seat, assuming the system hadn't magically kept me in a comparable seat.