Notice how much the geography of Hogwarts changed between films two and three. Could be because..
There will be a war. Hogwarts will be attacked. Giants will be involved. Hogwarts will be easier to defend. It will look better on film if Hogwarst is on a hill.
Also in this was I am expecting all four houses to join together against the threat. Some in Slytherin will have split loyalties. My guess is Malfoy will chicken out or reject Voldemort. He does'nt match up to his fathers bad ways - more like his mother.
The director added his own flavor to the movies, including changing the terrain around Hogwarts. I personally liked the change--made the movie more interesting and independent of the books--but many rabid fanboys/girls of Harry Potter were pissed off about it.