Mixed mode S parameter to 200Ohm system

Hi all,
Could anyone know the conversion of 4-port s-parameter to mixed mode parameter?

the case is that I have a differential in (2 ports) and differential out (2 ports) unit. I measured the 4 port s parameter into a S4p file. This s4p is measured on a 50Ohm system. I need to convert it to the mixed mode s-parameter on 200 ohm, what can i do?
I find the equation for 50ohm to 50 ohm conversion, but can't find the equation for the conversion between different impedance.

thank you for you reading.

Search Agilent web site. They have Application Note(s) for this issue. It can be done with transform S-matrix to T-matrix and then back to S-matrix if I remember it right. I used this techniques a couple of times, but do not remember all the details. Once we developed this technology and created Matlab file, we use it since that. It is based on Agilent papers I mentioned earlier. But because equations are hidden in the program I cannot see them and recall everything.