The best way for you to update to 5.1.1 is using sn0wbreeze: iH8sn0w.com
it will update, jailbreak and preserve the baseband all in 1 go.
the only way to downgrade your BB is using red sn0w (the version this thread is about)
if it doesnt work now you will need to wait for the update to be released.
once you have been able to downgrade the BB you will beable to use stock firmware again.
just remember to back up all your stuff before updating with sn0wbreeze because you will have to restore the phone.
once updated to 5.1.1 restore the phone from your backup and job done.