I am not certain that having the same seat number existing necessarily protects you on a plane change. I fear it can at times become a bit of a free for all while it all gets worked out. Had one recently where the system appeared to have basically “lost” everyone’s seat assignments when the plane changed, could swoop in and claim any seat

I have also noticed safety checks becoming less thorough of late. I did a flight where take off and landing the bloke next to me had a giant suitcase on the floor but not under the seat (we had an empty seat between us). I find it hard to believe it wasn’t visible and I would have tripped over it had we needed to evacuate, but remain on the floor it did. So not necessarily surprised by that one either!