On NW when I book a flight, I can select my seat on-line by viewing the seat map, i.e., I can see what seats are already occupied and select accordingly, and then anytime up until flight departure I can keep doing the same thing anytime I want by logging into my account and viewing my itineraries. That way if a nicer seat opens up I can grab it without having to call a human.

Unless I'm missing something, you can't really do this on-line on United, right? You can pick your seat initially, and make changes within 24 hours on-line, but in between booking and 24 hours the only way to change your seat is by calling a CSA, and even then you can't see the actual seat map for your flight to get an overview of how booked the plane is. Is this correct, or am I doing something wrong?