I haven't gone through the process but from I understand it's uncommon because it requires an opening in the medical school class. Given the low attrition rate at med schools, there usually aren't too many openings. Schools have former transfer applications which generally require good academic standing at US accredited medical school (some have more specific requirements such as you have to be married to a faculty member). If you can get your Dean on board with you about the transfer it makes the process easier since the transfer is a sort of a negotiation between deans. Oh, and it usually takes place between the 2nd and 3rd yrs since curricula at different schools don't overlap, and that's the natural break between the didactic and clinical yrs.
This topic comes up in the osteo forum once in a while. You may want to do a search there.
