I was checking my mom's flight earlier today, and moved her to a window seat that wasn't over the wing. I noticed that one flight in her itinerary was listed twice (YYZ-LAX), so I called in. They said there had been an equipment change, and he cleaned it up. This cleared the seat selection, so I had to do it again. Had I not noticed, I assume a seat would have been assigned at check-in.

If the system automatically handles an itinerary cleanup, might it automatically reassign everyone?

Sorry I don't have a straight answer, but if there's one thing I've learned, it's that you need to keep an eye on your itineraries, especially as you get close to departure