Applying changes to outline-minor-mode-highlight

[Heime]

To change the highlighting of headings one changes the value of 
outline-minor-mode-highlight.

But the settings are not taking effect unless I disable outline-minor-mode and 
enable it again.
How can I change this code so that the changes are applied automatically ?

(defvar hkfocus-setting 3)

(defun hkfocus ()
   "todo"
   (interactive)

   (pcase hkfocus-setting
     (1 (setq outline-minor-mode-highlight nil)
        (setq hkfocus-setting 2))
     (2 (setq outline-minor-mode-highlight t))
     (3 (setq outline-minor-mode-highlight 'append))
     (4 (setq outline-minor-mode-highlight 'override)) ))