AA changes flight time by 6 hours to a plane half the size?
 

Hi! Newbie here. Within the past week a flight I have in July from PHL to MSY has been changed twice. The first change is arriving in New Orleans at noon, when I was previously arriving at 6pm (no big deal-- extra time in my favorite city!), but the second change was then an aircraft switch from an Airbus A319 (128 seats) to an Abraer ERJ-175 (72 seats). Has this ever happened to anyone? Is this normal?

Welcome to FT. Yes this is very normal for AA, on a lot of routes like this they will switch between mainline 737/319/320 and 175 depending on the season, day of week, time, etc.

Yes, it's happened to lots of people (all of the people also booked on your flight!), and it's relatively normal. AA doesn't normally make a lot of changes only 3 months in advance, but there are extra schedule changes going on right now because of the grounding of the 737-8 MAX aircraft through the summer.
The E75 should be considered an upgrade from an A319, it's a pretty comfortable plane. That said, because of (1) the 6 hour time change, and (2) the change in operating carrier from AA mainline to one of their regional affiliates that operates the E75, you can request either (a) to change to another flight of your choice, or (b) a full refund. Up to you.

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.