Do you mean without turning it off or what? I think you will have to rerun the setup process. I never have done this myself, only have read on the forums few times. You can make a backup of your server and then give it a try. After a backup stop the server, edit /openfire/conf/openfire.xml and change the last tag to false, run the server again and go to the Admin Console. It will show a setup process again. go though it and specify different domain name. I think by doing this it should change all your users IDs, etc, unlike if you just change it in the system properties.