I've always seen it that the first half shows the oppression and danger and the second half the humanity and hope offered by the rebellion and the people around you. And I believe that this is reflected in the music.
I guess that someone has linked this already but it is one of the most amazing videos I have ever seen. The second half really is breathtaking.
http://www.youtube.com/watch?v=yV00UE_UaAY