Actually that's not the full picture. Any registered SBR can be taken off the NFA role by removing the part of the weapon that required it to be registered in the first place then notifying the ATF in writing stating it is no longer in the configuration requiring it to be registered. In other words you can remove the short barrel, replace it with a 16.5" one, then notify the ATF you have done so and request it be removed from the list. BTW do not keep the short barrel. The same would apply to those who registered their pistol braced rifle. Take off the brace, send a letter to the ATF you have done so and it is no longer in a configuration requiring it to be registered. Give it a few days to reach the NFA branch, then put the now legal pistol brace back on the rifle and do with it what you want.Until you try a sell it. No one is going to what to buy a registered pistol grip gun.
Disclaimer
I am not a lawyer offering legal advice. This is just my understanding of the NFA laws after many hours researching them.