def preference_change(name, group)
        [preferences_changed_group(group)[name], preferred(name, group)] if preference_changed?(name, group)
      end
def preference_change(name, group)
        [preferences_changed_group(group)[name], preferred(name, group)] if preference_changed?(name, group)
      end
Builds an array of [original_value, new_value] for the given preference.
 If the perference did not change, this will return nil.