How to edit a Certification's 'Reassignment' from: "Based on Enrollment date" to "Completion Date"

The LMS admin before me created a Certification that must be completed every 12 months. 

For some reason, he chose to set the Reassignment option to Based on Enrollment date. This makes no sense to me, since we want to make sure that the learners complete the certification every 12 months, based on when they last completed it... not when they last enrolled. 

So... I need to edit this option. However, when I pull up the Certification and click Edit... I can edit every field except for Reassignment. It appears to be hard-coded to stay at Based on Enrollemnt date. Is there not a way to change this to Based on Completion date?

If this is one of those situations where, once a learner has completed the cert, this options is no longer editable... does anyone have a work around for what to do in a sitaution like this going forward?