@
ocam
, I think you could preserve your baseband and then unlock the same way you already do.
I'm not 100% on this.
I did a phone a while back, I preserved the baseband/downgraded the baseband to a ultrasn0w unlockable one...
There might be a better way I dont know about yet though.
EDIT:
http://www.idownloadblog.com/redsn0w/#baseband I was right
