Describe the bug
Hook into SPELL_UPDATE_USABLE event.
ActionbarPlus still does not receive this event unless the macros also exist in the blizz actionbars (so sad).

Example Macro

For instance, when the target changes from harm to help, the icon should update from "Starfire" to "Regrowth"

/target [@mouseover, nodead, nocombat]
/cast [help, nodead] Regrowth; [harm, nodead] Starfire; Regrowth