How do you change an Individual assigned to an AD Group Assigned.

We have many individual assignments to applications for which we have AD Security Groups. Some admins have assigned the application manually BEFORE adding them to the proper AD group. If I remove the individual application assignment and then perform an AD sync the AD group assignment does not work. It will not come back as assigned just from being a member of the correct AD group. How can I convert the individual assigned to an AD assigned?