Yep. I've booked several flights originally scheduled to be operated by a 737-300 or a 737-400 and had it swapped out for the other on the day (as the airline operated a mixed fleet with both types regularly swapped around to better match demand and/or respond to maintenance issues). Also had a 757 swapped out for a 320 a few weeks prior to the flight. Or a 777-300 swapped out for a 777-200 several days before the flight. Even a 787 was swapped out for a 330. I suspect some of the changes are demand driven e.g. swapping to a smaller or bigger aircraft. I have observed this cross multiple airlines: Qantas, Air NZ, British Airways, China Southernm etc. Quite common especially for mixed fleet airlines.