I knew of only one transfer student at my school, so the incidence is definitely low. For a transfer between 1st year and 2nd year, there is practically zero chance because every school has its own preclinical curriculum/calendar and they don't want people to come in and make things harder for themselves and the student. They want everyone to make a clean transition from preclinical to clinical, which is why transfer applicants are required to have completed two years and COMLEX Level 1 (yes, they want to see your board scores).