Face made from surface is changing her boundaries

Hello,

Context:

     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:

     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?

Thanks!