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Changing the “Expiration” setting only affects future learning records. It does not affect the expiration date of previously issued learning records. To modify previous learning records, see Updating Learning Records After Changing Course Completion Settingsthe Expiration Dates
Changing the “Recertification” setting affects both past and future learning records. In other words, if you set a recertification window on a course that previously did not have one, all users, regardless of when they completed the course, will now be subject to the recertification window you specified.
The recertification rules you that you define in a course are hard rules. In other words, there is no way to override the rule on a per user basis nor make exceptions. If the course says that a user can only get recertified 60 days before their next due date, there is no way to let them start the course 61 or 62 days before their expiration date.