this is one from various feature of direct modeling, this command available from Unigraphics NX1 until Unigraphics NX6, the function of this command is make same flat or same position of face that have different hight,
in the direct modeling feature, Unigraphics give us to editing face and solid model directly like change diameter of cylinder, or arch face, change radius of blend directly and others although the part parameter is removed.
to begin direct replace face command click the picture like below
after dialog box appear select the face that you want to move, select third button then select the target face, click OK or Apply
to make tutorial more easy to understand i post video, hope this useful
Watch direct modeling replace face in How to Videos | View More Free Videos Online at Veoh.comother tutorial also available in the post
http://ugs-tutor.blogspot.com/2008/08/making-block-from-block-offset-ediitng.html