Forum .LRN Q&A: Is there an easy way to change roles (and permissions)

When professors are added to classes, they have the corresponding role
plus administrative rights (via permissions) on that class instance.
Is there an easy way to downgrade a professor to student or vice-versa
(upgrade student to teaching assitant or professor) so that
permissions and roles are automatically adjusted?  I can't seem to
find any API or a UI interface for this.