Yes, if someone else wants to Form 1 a previously and current SBR or SBS, they must have the current registrant send a letter to the ATF to unregister the firearm first; however, as I've pointed out and nauseam now, the new owner would NOT be illegally in possession of a registered SBR if it's not physically an SBR. They can legally own and use the firearm just like any other pistol or rifle. It wasn't illegal for the registrant to sell it nor is it illegal for the buyer to possess it.
Reading comprehension. I never said that the tax stamp disappears or the firearm stops being registered just because the stock is removed. What I did say and what is factual is that the NFA laws, regulations, and requirements disappear when the firearm is no longer physically meets the legal definition of a SBR or SBS. It's just like a braced or bare buffer tube AR pistol is treated as a hany when in handgun configuration and all state and federal handgun laws apply. If you put a 16" barrel on that same AR pistol, then all state and federal handgun laws cease to apply, and now state and federal title 1 rifle laws and regulations are applicable. It's the same exact phenomenal when it comes to SBR and SBS.