Some of us select less-popular departure times (e.g., 6 AM, 11 PM) because those tickets cost the least. Wouldn’t it be nice if the airline called you and said they had a more desirable flight time available, maybe with fewer stops, and you could switch to it at no extra cost?
Well, that is an option when American or United does a schedule change.
This article has the details, and includes the different rules that Delta follows. For American Airlines, you want a schedule change of 90 minutes or more. For United Airlines, you want an arrival time shift of at least 60 minutes.