thats also misleading. if the item has %ed, this % will stay the same, no matter what. normally any item that has such a property has maximum base defense.The only things that get rerolled will be strength/dexterity/level requirements and base defense for armors or base damage for weapons. What is being referred to is that some uniques, for example the HoZ or Vampire Gaze, can actually have a lower defensive value after they are upgraded. This happens when the exceptional version has a very high %ed and the elite a very low %ed.
+100% Enhanced Defense
Adds 6-22 Cold Damage - 4 Second Duration
15% Slower Stamina Drain
6-8% Life Stolen Per Hit (varies)
6-8% Mana Stolen Per Hit (varies)
Damage Reduced By 15-20% (varies)
Magic Damage Reduced By 10-15 (varies)