You must now uninstall Desktop Manager (through Add/Remove Programs) and then reinstall it.
I don't think you have to uninstall Desktop Manager. Re-run setup. It should have an option to modify the existing installation. Choose that option, and it will ask if you want to use redirector or BWC.
I have been toying with the idea of using Redirector to forward messages from client site that I'm calling home right now. I installed and uninstalled it (with the 4.0 desktop) yesterday.