If loads are looking good I think they do adjust aircraft schedules but they would have to consider how it would affect later services. They couldn't swap if it meant the next rotation originally scheduled in the larger a/c was full. Also with automatic checkin and seat allocation, changes could potentially mess those up and create more hassle than its worth.
HAJ doesn't do badly out of BHX and its not uncommon mainly in the summer mind you to see the E195 on the route.