Add role instead of replace role on update form

I have a site in which I am selling access to class information (if they sign up for the class). So I have set up a role for each class. This works great except for people who sign up for more than one class. If someone signs up for more than one, class, the user registration plugin replaces their role with the latest one instead of adding the role. I figured this out when I, as admin, reset my own role to subscriber after submitting the form for testing; oops. Is there any way to change this so people can have more than one role assigned by the same form and also have the update form add another role instead of replacing the role?