Here's my case:

• I have a JBoss AS 7.1 on which is already deployed an app compiled with Java7 (deployed in the standalone/deployments folder)
• In my workspace, I try to start AS with Java6. This fails (I get an error message dialog).
• I configure the server to use Java7 instead and try to start it again.

I does not complete the startup, it remains stuck at 15%, maybe because the previous startup failed and I can still see an "Update Modules status" job running in background