The good news is that AA can't charge you anything extra for the nicer aircraft. With 2-2 seating in steerage, it is far nicer and less cramped than what you had/

As to the broader question, yes, AA and other carriers do this fairly regularly. For AA, the -MAX situation means that it has had to do a lot of schedule and aircraft juggling and likely found a flight with a low load which can be performed by the =175 while the -319 will perform some other route.