Most airlines would only have one of Airbus A320 series/737 series, so if you get a ticket for an airline only flying the former, it's highly unlikely you'll ge

Most airlines would only have one of Airbus A320 series/737 series, so if you get a ticket for an airline only flying the former, it's highly unlikely you'll get switched to a 737 (Max or otherwise). The exception is code sharing/wet leasing/airline alliances (e.g. you might get an Iberia plane when booked with British Airways).