I don't know of a "migration" guide, so it is "try and error" ;-).
But some things to start looking at:
-Check the list of supporting JARs bundled with your app. You must remove all weblogic specific JARs, there should be no XML parser included (e.g. Xerces), there should be no J2EE jars and no webservice JARs. All of those are provided by JBoss. If you are not sure about the file list, you might post it here.
-do you use Weblogic specific deployment descriptors? If yes you have to find the JBoss equivalent files.
Probably you can keep your app at Java 1.4 for the first step.
Try to keep the steps as small as possible: first migrate the existing app to JBoss, then change to Java 1.6. And if all this is done, you might try to "upgrade" it from J2EE1.4 to JavaEE5 (this might be the biggest part of work).