If I have to take a course that has other courses as pre-requisites but my certifications for those courses are still active, do I need to complete them again prior to taking the course whose certification is expiring?

Pre-requisite courses for a higher-level course (i.e. Plant I and II for Mix Design) do not need to be completed prior to that course if those certifications are active. This means that you do not need to take Plant I or II before taking Mix Design if your Mix Design certification is coming due but your Plant I and II certifications are still active.