There is just an xp decrease when you fight those monsters with your highest combat skill, not including defense of course. So let's say you have 99 strength, 60 attack, and 99 defense. If you use strength to fight a guard, you'll see a noticeable decrease in your xp. if you switch to an attack style though, you'll get normal xp. Well, maybe normal xp.And yes, monster levels will change with this update.