How often/common does united swap an A319 for a B-757 on ORD-SFO?

Usually airlines only swap airplanes if they have a large number of bookings. In your case United is swapping there A319 for a B-757 because there is a large amount of passengers. Airlines rarely do that and will also only do swap planes if they are having some type of problem with the plane there suppose t to take. So airlines pretty much rarely swap planes. I don't believe your seat selection will change because all planes are the same its just that some have more seats then others so your fine.