Yes, or you can do it online. There should be a travel tools button near the top right, and you want to choose Change Air Reservation. You just follow the steps from there to revise your reservation to the new lower prices. The excess points go right back in your account!
No kidding! Now that I've done it once, I want to book with points always! I am not used to being able to make changes to flights but it really came in handy this trip.