When Bush leaves office in 2009, the U.S. will get a whole new set of leaders at the top level. New Secretary of Defense, Secretary of State, Homeland Security Director, etc. A lot of people will be walking out the door when his presidency ends. The new president will then appoint new people to all of those vacant cabinet positions. These people have enormous power (and responsibility).
So, there actually is a lot that can change when a new president comes to power. Look at how great things were under Clinton — and look at things now. Complete disaster.