Users who remain homed on Office Communications Server 2007 will experience no change, because they will continue to use the same browser-based client. If you are providing a single access URL for all users, then users of the 2007 release will now use this URL to sign in to Communicator Web Access. When the new server redirects the user, he or she will be prompted to sign in again.
Users who are homed on Office Communications Server 2007 R2 will have access to whatever new features of Communicator Web Access you have enabled.