I have always been proactively rebooked. Sometimes this does not work in your favor. I have shown up to my original flight 10 min before departure and they have given my seat away.
Does the proactive rebooking feature apply to all travellers or just elites? Also, AC doesn't double protect flights for passengers (i.e. hold a seat for the original flight whilst rebooking them on the new flight)? I've heard stories UA doing this for their elite travellers a couple of times.
-James