Be careful when editing the Enrollment Model on a Program that has current enrollments. If you change a program's Enrollment Model, anyone who is enrolled in that Program who has a Payment Plan attached to that Program, that Payment Plan will disappear from that Enrollee's account.
If you need to edit an Enrollment Model, be sure to do the following:
No one is currently enrolled in that Program
No one has current Payment Plans
If you can't do either of the above, then you have a few options:
Manually add back all Payment Plans to each individual account after the Enrollment Model is changed
Duplicate the Program first and then change the Enrollment Model. Then re-enroll everyone who was enrolled in the Program.