Jzar,
I was going to use UPDATE_SHAPESHIFT_FORMS, but
WoWProgramming said that it did not apply to stances/presences. So I asked here, having run out of self-help options.
I took Xubera's advice and found a secluded mountain top to run /eventtrace. Turns out you were right & WoWProgramming was not!
So for all who read this, UPDATE_SHAPESHIFT_FORMS does indeed fire after changing presences.
Thanks all!
Cormanthor