The transmission flush involves using additional fluid pushed through the transmission to remove the much more viscous “gunk” that tends to build up between all the parts over time. The fluid change just involves draining the transmission fluid and replacing it with the appropriate volume. In a lot of cases, mechanics will advise against a flush if the car has a lot of miles (like 175k+), especially if the fluid hasn't been changed regularly. The theory is that the “gunk” is the only thing keeping parts where they should be because of all the wear. You can easily find anecdotes from mechanics involving a high mileage car getting a transmission fluid flush and fill, then shortly thereafter it decides to destroy itself from the inside out.