This happens a lot when your flying with Alaska or Southwest. Both airlines have significant MAX fleets and will change the dispatch schedule for aircraft fairly often, in my experience on Alaska it happens about 10~20% of the time.

Also, Southwest does not assign seats and Alaska does not have a difference in seats between their -900 and MAX 9 aircraft. Although with Alaska I have had my seat reassigned because they changed from a -900/MAX 9 to a -800 before.

Happens quite often in airlines that operate a range of types. E.g. Airlines that operate both the 320 and 321, 777-200 and 777-300, etc often swap aircraft between pair of similar types but slightly different number of seats to match demand but still draw from the same pilots/crew due to common type ratings. But I've also had 320 to 777 swaps, 757 to 321, etc do it does happen especially on routes where they operate a range of different aircrafts. Yes some tickets may need to be reissued but that's better than cancelling the fight where all the tickets have to be rebooked onto other flights. Some changes are due to low demand so in those cases no ticket reissue are needed. It also depends on the airline, you'll see swaps more often on airlines that operate many different kinds of aircraft and also depends on the route as some routes can only be served by a particular aircraft (e.g. ultra long haul routes, airports with short runways, etc) while other can be served by a wider variety of aircrafts lending itself to operational flexibility. YMMV.