How to update a "Parent Policy" via a "Policy Template" ?

To clarify a bit: Parent policies ONLY exist for the purpose of creating child (security) policies which can inherit the settings of the parent. Templates (Comprehensive, Fundamental, OWA, etc.) exist in parallel to Parent policies. If a template is modified, the modification does not affect policies created from it in the past. If a Parent is modified, then that modification will propagate to all its child policies based on inheritance rules.