One of the areas that has been an issue over time is taking an application, say deployed to jboss 3.0 or 3.2.3, and try to upgrade it to jboss 4.0.5. Or tomcat 4 to tomcat 5. Or any upgrades at all.
Real-world experience with struts (1.0/1.1)/JSP sites have several challenges in upgrading. Whether they are container based or implementation base, I have never had success with 'easy' upgrades.
JSF appears to have similar issues. I myself kept running into performance issues everytime I've attempted a JSF implementation, so relying on this post to confirm similar issues: http://jsfunit.blogspot.com/2010/12/jsf-on-jboss-as6-final.html
Now, onto one known savior - GWT is upgrade compatible. I have successfully upgraded 1.3 to 1.5, 1.5 to 2.0, 2.0 to 2.1, and 1.3 to 2.0 (I haven't tried direct to 2.1). The only upgrade issues were 1.3 to higher versions dealing with RPC changes.