I recentlly talked with an admissions officer who said that it was actually quite difficult to transfer between the 5Cs. The officer's at each school are very wary when they get transfers between the Claremont schools because of the consortium itself. In their minds, why would a student want to transfer to say, CMC from Pitzer? The Pitzer student can already take courses at CMC, use the same library, participate in the social life. Unless you make a very compelling and legitimate case for why you'd like to transfer, the request will most likely be denied according to this officer.
I'm sure the officers make assumptions about wanting to change schools for reputation or "name" reasons; even so, the student is already in Claremont... I believe that is the mindset.