Hi Lucy
Unassigning the user will place their certification into history - when the user is reassigned the completion record will be restored from the History - saying they have completed this.
You want the certification status to be Not certified and certification path to be Certification with a due date when this needs re certifying.
Using the editor option 1 Certification completion date should be 'Newly assigned' - this should make the Certification status 'Not Certified ' Change In progress to 'No' as no progress has been made yet. Adjust the Due date to when re certification is required.
Regards