They are actually quite flexible these days, especially since they dropped change fees for most Main Cabin tickets. I recently had to move a flight up by a day because a meeting ended early. I was able to see all my options right on my phone while I was still in the taxi. In my experience, checking the "Change Flight" button in your trip summary is the best way to instantly, or modify flights to a more convenient time without any major hassle, provided there’s a seat available.