IMHO it doesn’t do any good to speculate on theoreticals, but it won’t be the first time ATF has changed their minds about the legality of something after approving it.
Wasn’t the Akins Accelerator reversed because the design was changed between ATF approval and production? Same with the Sig Brace where it was approved with the strap but if they’d been produced without it they would have changed their decision. They make decisions based on design
It depends on the particular wording of the ruling. Things don’t have to be grandfathered in, but they often are. Are the Sig arm braces serialized? That’d make it a lot easier to grandfather them in, but not having them serialized doesn’t necessarilly preclude grandfathering.
The courts determined the ATF did not have legal authority under NFA law to determine if something was a machine gun based on the date it was manufactured. So once a given firearm is determined to be a MG, all are MGs regardless of when made.
This is why when they determined Street Sweepers were DDs, they did not grandfather. All Street Sweepers were DDs and they offered a grace period to register them.