It is EXTREMELY difficult. The schools do not want to ruffle feathers at other schools. Remember this is a mafia of sorts just like the NCAA and the Ivies. If there are disciplinary issues it tends to be movement to a lower tier school. If it is fit or other issues it tends to be at schools to which someone may have been admitted previously. There are athletic mis matches: i.e a very good hockey player left Groton and went to Salisbury because of the hockey program.