Remember that "Change" slogan? Here are some thoughts on that, between other things:
Removing him will be unsettling to those in the Pentagon who realize he is being let go for advocating their views as well as to those throughout the administration who will worry whether getting rid of him will be seen as enough. The real changes needed are at a high level in the White House, in the team immediately around the President advising him on national security.
Unless those changes happen, view the Hagel move as a distraction, as an effort to create the illusion of change where a shake-up is needed.