You still have to downgrade, but you don't need to be unlocked. Just download the .30 firmware, pop it on an SD and it will notify you to downgrade. Also, the unlock tool still works on JB if you still wish to unlock.
I recently bricked my device and had to do this, so it's certainly possible. I'm not unlocked just yet, but people say the unlock tool does work.
EDIT: I just used the Unlock Tool and can confirm that it works with Jelly Bean just fine...now to hope I don't regret unlocking!