How do I make change to XXX-font-lock-keywords (typically using assoc plus setcdr) affect the fontification in XXX-mode? It doesn't suffice to simply change the variable to make the change have effect.
The best way, as a normal user, to modify font-lock entries is to use the function font-lock-add-keywords and font-lock-remove-keywords. For example:
(font-lock-add-keywords c-mode
'(("\\<\\(FIXME\\):" 1
font-lock-warning-face t)))