A floor panel assembly (used as replacement floor panel or in a new installation) includes universal quick-attach corners that mates with support stands and stringers of an existing raised floor system to provide leveling. The panel assembly includes a top plate (apertured or not) with corner structures including a post retainer and interconnected down flanges that support the top plate inboard of its perimeter. In one form, the universal quick corners each includes an arrow-shaped extension plate attached to the post retainer and extending toward the top plate's perimeter, and includes a body with multiple thickness pads selectable to support the extension plate at selected height. Thus, the quick corner holds an upper surface of the top plate level with an upper surface of adjacent existing floor panels of the existing raised floor system. In another aspect, only the body is used at each corner (without the extension plate).