Competency Based Performance Appraisal - Change of Supervisor

Hi, We are developing our performance management system using competency based method. As part of our process there is a possibility that the supervisor for a particular employee can change during the performance appraisal timeline. How is this handled? After the supervisor is changed and employee/supervisor relations built again, we can no longer access the comments entered by the previous supervisor. How can the new supervisor access the old supervisor’s comments and contribute to the employee's appraisal.