Please see: https://web.respondus.com/aleks/

Please note that once “student purchase” or “institutional licensing” for Respondus Monitor has been selected for an ALEKS course/cohort, you cannot switch to the other option. This can only be done with future ALEKS courses or cohorts.

This means that the license that applies for the course/cohort is the one that was active at the time Monitor was enabled in the course,  and cannot be changed after the fact. 

If you have switched your license from "Student Purchase" to one that links the course/cohort to your Respondus Monitor license, only brand-new courses (where the dashboard has not been touched and no exams have been enabled for Monitor) will use the new linked licensing.

As a test, you can set up a new course (or make a copy of an existing course) and then enable Respondus Monitor for an exam in that course.  The student purchase option will no longer be triggered, and the Respondus Monitor seats from your institution's license will be used instead.