Normally, you're automatically rebooked when an airline adjusts its schedule. Airlines do this several times a year, but not all flights are affected, you were just "lucky" enough to be on one of them.

However, you should periodically check your reservation to see if anything has changed. A common occurrence is for your seat reservations to be switched, leaving your party scattered around the plane.

It's also possible that the change will have a new flight time that's unworkable, or a connection that's impossible. I had a change on Delta that left 15 minutes for an international to domestic connection. In these cases, you have to call and get it fixed.

At the very minimum, you should check a day or two before for the exact flight times in case they've changed. (Back in the Elder Days before the internet, we had to call the airline the day before to confirm the flight times.)

It would be nice if the airline would call or e-mail you, and sometimes they do, but you can't count on it.