It doesn't have to be "removed from the registry" either. You could register an Encore as a SBR and use it with a long barrel or (as legal in your state) as a handgun. Registering it as an SBR doesn't mean it has to stay in that configuration forever, it means it's ALLOWED to be in that configuration. If you sell it, you'd have to go by the NFA rules but you can use it yourself however you'd like.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.