I checked in online and, as booked, got a seat on a 747-400. On arrival at the airport my nicely home printed boarding pass was torn up and I got a new seat on the 777-300 that actually operated.
Same year, when we boarded the outbound A320 the flight deck advised us this wasn't the plane we should have used but one switched from another flight. In that case I'm pretty certain that by swapping the two A320s the airline had 2 slightly late departures rather than one on time and the other sufficiently delayed that EU compensation might have been due.
As others have said, quite apart from IROPS I've just described the same flight can be scheduled for significantly different planes on certain days because of planned loads. Bear in mind that it can be cargo capacity that drives some changes, not just passenger loads.