Leper is the character that I think changes the most.  Basically, he goes nuts.  Usually, Leper is quiet and particularly eccentric for a boy of his age.  He enjoys nature and skiing rather than sports and socializing, but this may be because he is an outcast among his friends because he has slightly different tastes and interests.  Due to being socially isolated to a point, Leper decides to enlist in the ski troops, making him the first of the boys at Devon to join a branch of the military.  He does this, I think, to feel like he is important, serves a purpose, and is evolved.  Leper cannot handle the stress of service, however, and goes completely insane.  He goes AWOL after he realizes that he will be served with a section 8 discharge, dishonorable to say the least.  If this happens, he will not be able to get a job anywhere, so Leper goes home to Vermont.  I think this is a pretty significant change myself, so there you go.