Is there a requirement to fly the plane advertised? In other words, if I book specifically a non-737 800 Max plane, and the airline switches, can I get my money back, or make them fly me on a different airline?
It's not something an airline can do hastily, as different aircraft (well, at least those of differing type rating) require different crews to operate. So the airline will have to grab a totally different crew to match their replacement aircraft.
Before the 737 Max got grounded (after the 2nd crash), I remember reports that some airlines allowed passengers to rebook their flights for free (to end up on a different plane). I doubt they will continue doing it once the 737 Max is allowed to fly again. Eg, Southwest is too dependent on it.