Hi Sunil,
You can look at the wsdl file of the web service. Usually it can be
retrieved by appending ?wsdl parameter to the URL of your .asmx file. For
example http://localhost/webservice.asmx?wsdl. Then you can use wsdl.exe
tool to create proxy class that calls the specified service. All you need to
override the existing web service is to change the URL to the existing web
service with a new URL to a new one. The new web service should implement
the same methods and after your code gets called you can pass the control to
the old web service.

Regards,