Changing a class from Public to Restricted to allow me to add learner access

We have a requirement to add learner access to allow self enrolment on a class that has been setup as not being Restricted and so the class is stuck at requiring line manager approval.  I have written a data-fix which sets the public_event_flag to 'N' thus allowing me to set learner access and new enrolees to self-enroll.  Without the data-fix i cannot add learner access and i receive the error "You cannot change the Restricted status of this class. The change has failed at step 1. Steps 1. Learner enrollments already exist for this class. 2. Class associations or restrictions