I book many of my trips way in advance, so I'm used to getting e-mails that a flight has been moved up 10 minutes, or pushed forward half an hour—no biggie.
But today I got one from American Airlines, changing an itinerary to Portland, Oregon to start 5 hours later and arrive nearly 3 hours later, too late to check in to my B&B. Problem's been solved; I called AA and they moved me a couple of hours earlier than the original.
What I learned in the process, however, is that when a flight is cancelled, AA and most airlines won't book you on the most similar itinerary; they'll book you on the "next available," because that's what it says in each airline's Contract of Carriage, which covers your rights...and theirs. They won't put you earlier unless you request it, and in doing so, implicitly waive the right to "next."