Welcome to the JavaRanch.
Upgrading is tedious. I moved a large application from WLS 6.1 to 8.1 and it took
months, mostly due to problems migrating security realms. You can have a look at the
WLS 8.1 Upgrade Guide and see if that helps. The bottom line is, the
J2EE specification has undergone huge changes since 5.1 and you application will need to be changed to comply.