Well, everyone has to start somewhere! You should be able to do HEDIS, but the early stages might be tough going. Your behavioral background probably doesn't give you much experience evaluating lab values or slogging through the nuances of terminology, etc. The actual work itself, finding the data and retrieving it, should be easy enough. Once you understand what it is that you are looking for, and where to find it, you should be fine. Best of luck, this is a brutal job market.