This is outright FALSE. The ATF in their final rule said that you have 120 days to SUBMIT a Form 1 for your braced pistol, and you must use a special eForm 1 on the ATF eforms website (eforms.atf.gov) in order to have the normal $200 tax waived.
After 120 days, AS LONG AS YOU SUBMITTED THE eFORM 1 PRIOR TO THE 120 DAY DEADLINE, you're still legally in possession until you either receive your approved form and stamp, or your application is denied.
If your application is denied, you only need to remove the brace and permanently alter the pistol so the brace cannot be readily reattached. (This usually means removing a carbine buffer tube and replacing it with a dedicated pistol buffer tube for AR platforms.)
Watch this 16 minute video from an ACTUAL LAWYER WHO PRACTICES 2A LAW for more: https://youtu.be/iGdjQ4Iylzo
There may (or may not) be additional factors involved. For example, it is possible the FBI may fail to complete the required background check within a specified period of time, and the application may enter some status that is neither "approved" nor "denied". In addition, many AR pistols equipped with braces already have standard (short non-adjustable) pistol buffer tubes/receiver extensions. Finally, I have seen it stated repeatedly that the ATF's position is "once a rifle, always a rifle". Whether that is true or not, or true in some cases and not others, I do not know (I am not a lawyer), but it seems that the point in time at which ATF considers or considered any specific brace-equipped pistol to be an SBR may influence the ability to convert that specific firearm to a legal non-braced pistol, rather than to a legal non-SBR rifle (with a barrel 16" or longer).
I am not trying to pick a fight - just suggesting that when it comes to federal firearms laws and regulations, things are sometimes not as simple as they appear.
;^)
Is it 4 million or 40 million potentially affected parties? Either way, the ATF and courts don't have the bandwidth to deal with this level of stupidity from ATF. Are there enough 2A competent lawyers to hire if you guess wrong?
I have one AR lower with a smooth, non-carbine buffer tube for use with pistol uppers. I don't own any braces. The ATF tax stamped SBRs have real rifle stocks.
Personally, i would like to see ATF agents die by the dozens trying to enforce this lunacy. I would stand up and applaud.