take it to a non-SBR state? At that point it's technically not an SBR. But I wonder if you break any laws bringing the registered lower into a non-SBR state.... I'm military and move a bunch, just curious.
I can't give you a definitive answer other than to check the FAQs on the ATF website. I THINK they touch on this subject.
Something else to think about (and I've wondered this myself): Assuming it's ok, if you leave the state with a 16" upper on your registered lower and assuming it's your only registered lower, are you guilty of constructive intent if you leave your short upper in the safe with your non-NFA lower?