I have a different take

If the ATF changes their mind and calls them MGs, there will be no grandfathering short of a court order. This isn't your father's Tax and Trade Bureau. Today's ATF would show up, take the stock and leave. If you put up a stink, they'd charge you with possession. Same thing they did with the Cole's Uzi parts kits awhile back when they retroactively decided the receivers weren't demilled properly.

The Striker/Street Sweepers got grandfathered because there's no making ban in effect for DDs.