It depends on the apps you're running. If it's a simple webapp with session replication, a migration should be easy. If there are beans involved, it might be a bit more complicated.
Before you do that, you could try upgrading to JGroups 2.12.x, but I'm not sure it'll work with JBoss 4.x. It would only take you 5 minutes to find out, so perhaps that's the better avenue to choose at this time.