Here's a nice link that goes into detail...
When you change practices
Consider this scenario: Suppose you leave the practice where you have been working for a number of years to join a new group in a nearby community. Some of your patients transfer their care to the new practice and see you within three years of their last visits. You would report these encounters using an established patient code because, although you are practicing in a new group, you have provided professional services to the patient during the last three years. Note that whether the patient has transferred his or her medical records to your office and how long you may have had those records is irrelevant. The amount of time that's passed since your last encounter with the patient is the determining factor.
http://www.aafp.org/fpm/20030900/33unde.html#Box_D