If you used Jonas application server specific APIs then obviously you'll need to convert these to use equivalent APIs on JBoss.
If your application is a vanilla J2EE app and you are not using any other 3rd party APIs, then it should be a relatively easy job to deploy it to JBoss.
Why don't you just deploy it (do you have it bundled as war and/or ear files?) - drop them into a deploy dir on JBoss and see what happens... the quantity and type of exceptions you get will give you an indication of how much work you'll have to do to get it to work on JBoss.
It is deployed as an ear. The problem that I see is that all of the vendor specific deployment descriptors will need changed. I have been crusing this site for a while and can't find a tutorial, or an example that shows what the deployment descriptors look like.
Do you know if there is any documentation on the JBoss ejbjar ant task.
I also am looking for some document on what has to get included in the client jar files, in terms of stubs and skels.
I guess I will have to buy one of these books before I spend the the time porting.
If you download and install JBoss you will find DTDs of the deployment descriptors in the docs dir.
Buy the download docs, and these should give you all the info you need.