Hi, I don't think that this is such an unusual case in a company environment. We are right now running into the problem that we want to upgrade to JBoss 4.0.4, but that we have to support client applications which are using JDK 1.3 and which definitely cannot upgrade the JDK right now. So I would be very interested in any ideas how to solve this problem! The constraints I see are that the client apps cannot use the new jbossall-client.jar, since they get an "unknown major.minor version" error. And if they use the old client JAR, they do not know the new SingleRetryInterceptor class. The Wiki (http://wiki.jboss.org/wiki/Wiki.jsp?page=RetryInterceptor) shows how to change the RetryInterceptor, maybe one could use this to install the old RetryInterceptor? But if this works, what about serialization? Maybe I'm just running into the next problem thereafter? Though there is another Wiki page (http://wiki.jboss.org/wiki/Wiki.jsp?page=JBossVersionCompatibilityMatrix) showing some options about serialization compatibility. Would this solve the problem finally, or is the next problem just lurking behind? I could, of course, always install an old JBoss (3.2.x) and connect to the new JBoss using HTTP or similar. But then Good Bye to performance and fail-over. :-( Thanks a lot for any help! Martin View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3984253#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...