Face made from surface is changing her boundaries



     I have solid and all of its faces have CN continuity except two faces which have C0. I need to make those C0 faces become at least C1. So, i tried to approximate surfaces of C0      faces  with GeomConvert_ApproxSurface() and then make new faces of them. However, this method works not in all cases. 


     Problem is that faces obtained from approximated surface have larger boundaries than initial face. Even setting of Umin,Umax,Vmin,Vmax while making face from surface doesn't work. In addition, if i try to make new face from INITIAL surface of initial face, i get the same problem.  

Please, help me to understand why do boundaries change? Or maybe how can i change the continuity of face in different way?