i forgot to mention that the CNFE is thrown during startup of jboss!
recompile all your code with new jars from jboss. And it should work.
what kind of jars? seam or jboss as ?
I still dont know, how a different annotation (stateless) could cause this cnfe?
when you build your app you probably have some jboss jars in your lib. Replace all jars that you had from older jboss with new ones.
Do clean compile and it should work.