This invention is a system and method for an interworking function that handles communications between Rich Communication Services (RCS) Pre-Universal Profile (Pre-UP) and Universal Profile (UP) clients, and their Controlling Function and Participating Function service points. The interworking function supports the interworking for Pre-UP and UP clients in the following scenarios: 1) CPM Standalone Messaging and CPM 1-to-1 Chat, 2) CPM Chat and IM/SIMPLE Chat, 3) File Transfer by MSRP and File Transfer by HTTP, and 4) Open Group Chat and Closed Group Chat. The interworking function performs the necessary SIP message conversation and/or headers and message bodies manipulation, acts as MSRP client and server, acts as a HTTP server to store file for file transfer interworking, and acts as message proxy for Controlling Function and Participating Function.