I have posted the link and quoted what the proposed rule change states as far as what to do with a braced pistol once the rule change goes into effect in several threads here. And others have posted them too.
But basically you have several choices:
(1) Permanently remove or alter the “stabilizing brace” such that it cannot be reattached, thus converting the firearm back to its original pistol configuration (as long as it was originally configured without a stock and as a pistol) and thereby removing it from regulation as a “firearm” under the NFA. Exercising this option would mean the pistol would no longer be “equipped with” the stabilizing brace within the meaning of the proposed rule.
(2) Remove the short barrel and attach a 16-inch or longer barrel to the firearm thus removing it from the provisions of the NFA.
(3) Destroy the firearm. ATF will publish information regarding proper destruction on its website,
www.atf.gov.
(4) Turn the firearm into your local ATF office.
(5) Complete and submit an Application to Make and Register a Firearm, ATF Form 1 (“Form 1”). As part of the submission, the $200 tax payment is required with the application. Pursuant to
27 CFR 479.102, the name, city, and state of the maker of the firearm must be properly marked on the firearm. All other markings, placed by the original manufacturer, should be adopted. Proof of submission of the Form 1 should be maintained by all possessors. Documentation establishing submission of Form 1 includes, but is not limited to, eForm submission acknowledgement, proof of payment, or copy of Form 1 submission with postmark documentation.
This information comes directly from the proposed rule change as posted on the Federal registry. And as you can see there is no distinction between a lower or pistol purchased with a brace pre installed or a brace added after the pistol was purchased or built.
Again we really do need to wait until the official rule change is actually published to see what it says. And as far as I know that won't be until December 2022.