Hi Ian
If you edit the certification and then go to the 'Completion' tab, you can edit the completion records of individual learners by either making them newly assigned and setting a new due date, or choosing the relevant Certified option and updating the window open date and the expiry dates. This seems to hold for us.
We've only ever done this for a few individual learners, so it is a manual process - not sure if you could do any bulk uploading with this.
Hope this helps.
Jane