Well, it's a big project, and I doubt you'll get much help on every detail of it here.
With regards to migration, the easiest thing is to simply throw your code on to the new JBoss and see what breaks. Now, the great thing about JBoss is that it is fairly backwards compatible. The trouble areas are classloading and the areas which do change are the server configurations and security configurations.
It's better to jump in and post questions if you are stuck, than expect us to anticipate every possible problem.
Thanks for the reply. Yes it is big and I certainly didn't expect details here. You have good advice and this is what we are doing now so I will post if I come across any issues I can't solve. Thanks.