Change NetName

I want to change/replace the NetName in the capture design through TCL Script. So far i've been able to change the PinNames of any particular part and the names of OffPageConnectors of whole pages. I am getting the PinName and its corresponding NetName through $lPin. What I actually want to do is to add the PinName to its corresponding NetName with an underscore "_" between them. The Net Name is not being changed but if I try to change/replace the PinName which include the (NetName_PinName) it just works fine.

Please give me respective suggestions.