[jboss-as7-dev] JBOSS Upgrade still compatible ?

Hello, I was assigned to upgrade a bunch of servers to the newest JBoss version. These servers are RHEL 4.7 , RHEL 4.8 and RHEL 5.5 I don't want to mess up so I was wondering if JBoss is still Platform-independent here or is the JAVA version gonna cause a problem? Also can't I just "update" instead of "migrating" all the applications to the new JBoss? ( That's just something I found during some research I made) I hope you can help me !