You can do this.
1. Make sure you that the jar files you need to update are loaded into classpath using QuickServer from "application-jar-path" in XML and are not in your startup/command line/boot classpath.
2. When server is running, just replace the jar file.
3. Now invoke "restart server" using QSAdminServer (via telnet or shell or QSAdminGUI)
The re-start will not kill connected clients. The connected client will still continue to work under the old logic/classes. But when you now connect the new client they will use new class from the jar that you just updated.