code: def participation_policy_changed(ob, event):
      
      workspace = IWorkspace(ob)
      old_group_name = workspace.group_for_policy(event.old_policy)
      old_group = api.group.get(old_group_name)
      for member in old_group.getAllGroupMembers():
          groups = workspace.get(member.getId()).groups
          groups -= set([event.old_policy.title()])
          groups.add(event.new_policy.title())
docs: Move all the existing users to a new group