Not quite. Under some circumstances, a registered gun can be "unregistered" and removed from the NFRTR. And a registered machinegun can be converted to a semi-auto, or even destroyed; in the latter case, it can be removed from the NFRTR on proof of destruction (not deactivation). What can't be done legally is for an unregistered machinegun to be converted to semi-auto with the idea that it then will be legal.
Jim
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