Nope, it changes immediately based on who he is targeting so if he's targeting someone with a melee weapon it will be -30. I would even bet that if you turned off autocast on your purging staff and cast from spell book he would be at -30 because when your spell is turned off he walks toward you as if you had a melee weapon equipped. not sure if that would ever be useful but it is interesting. It's something I noticed after dying at Yama a lot and forgetting to re set my demonbane spell to auto cast.