In this example, we enter an existing patient’s file and update his demographic information.  That update will be reflected in all future records created on that patient, but this particular patient’s records will also be tied to prior visits before this update was made.  In other words, the patient has a unique identifier in the system that always ties his/her records together.  A common example of a need for this kind of change is when a person gets married and her last name changes.  By updating her demographics to reflect the change, all future records are created under her married name, but all her prior existing medical records remain a part of her medical record as well.