There are not many flights and routes that are not sold out often anymore... Also, putting a different plane on the route is a complex equation that needs to factor in the whole itinerary of the plane and crew that day. I have seen generally full flights downsized from a 757 to a 737, which would seem counter-intuitive, but in the grander scheme of things the only way airlines make money is by capacity scarcity: running full planes filled with people who did not all buy at a low fare.