There are always issues involved when moving from very old versions (Tomcat 1.x) to more recent versions (Tomcat 5.0.28 ships with JBoss 4.0.2). This might be your first stumbling block.
Secondly, if your app is a simple WAR, you should be able to drop it into the DEPLOY directory without a hitch. If, however, you are taking advantage of the additional J2EE technologies (JMS, Datasources, JNDI, etc), you might want to look into wrapping your WAR up in an EAR. Our website offers a free download of chapter 3 that discusses exactly how to create an EAR. (http://www.jbossatwork.com/articles.html)
I thought most people develop in Tomcat in exploded mode without putting everything into a single WAR? At least thats how I used to develop. If so, your first step is to create a build process (script) that constructs WARs/EARs for you to use on JBoss
If it's a web-only application, then you don't need an EAR. Just configure your DataSource (if you're using a database) in *-ds.xml file and drop in the WAR, and you'll be fine. It shoould work OK on JBoss.