Herald of things to come
Yes, it will become a Zakarum Shield but keep in mind that the defense may not necessarily go up by much. A Gilded Shield has a AC range of 144-168 and a Zakarum Shield has a AC range of 169-193 so if your HOZ had a base AC of 168, you may only gain 1 AC point to have the +defense% added to. Plus, keep in mind that the level reqs and the strength reqs will also go up so make sure you have at least 142 strength and be level 61 before upping your HOZ.
But, you have another problem...one that's been addressed by the above posters. You are on NL which means you cannot increase your unique items beyond exceptional type and thus this discussion is sadly a moot point.