I have upgraded several versions of JBOSS for my web application without any problem, but the last upgrade from JBOSS4.0.4 to JBOSS4.2.2GA obliged me to change some code, especially in the web configuration.
JBoss4.2 contains its own JSF implementation, and my JSF pages using JSTL tags (foreach ..) did not show my tables anymore. JSTL tags could not access the JSF beans.
To solve this problem I had to convert several beans from request to session, and the beans had to be used/initialised in a previous page. With this method, the JSTL tag could access the data, and the web display becomes as it was before.
I do not know if it is JBOSS implementation or the new JSF implementation, but it could be useful for anyone else using those three technologies together and wishing to upgrade to JBOSS4.2.2.