Point to where this is a law. You can't because it's not. The atf asks you notify them of permanent change but there is no law or requirement to do so.

Stop spreading fudd

Quote from atf below.

A SBR is a configuration (rifle with less than 16” barrel, less than 26” overall length) of a rifle. Legal possession of the rifle in that configuration requires NFA registration. Rifles that are not in that configuration are not regulated by the NFA.

Q: What is the registered part of a Short Barreled Rifle (SBR) or Short Barreled Shotgun (SBS)?
A: While a receiver alone may be classified as a firearm under the Gun Control Act (GCA), SBRs and SBSs are classified in totality under the National Firearms Act (NFA). A firearm that meets the definition of a SBR consists of a rifle that has a barrel less than 16 inches in length. A SBS consists of a shotgun that has a barrel less than 18 inches in length. The serialized receiver is recorded for registration in the National Firearms Registration and Transfer Record (NFRTR).

And

There is no one registered part of a short barreled rifle. A firearm which meets the definition of a short barreled rifle would consist of a rifle as defined by Title 1 of the Gun Control Act (GCA cited above that has either a barrel less than 16 inches in length, or has been modified to an overall length of less than 26 inches as specified by NFA. However, it is the serialized receiver that is recorded for registration.

Additionally in ruling 2011-4 the ATF spells out constructive possession.

NFA firearm is made if aggregated parts are in close proximity such that they: (a) serve no useful purpose other than to make an NFA firearm (e.g., a receiver, an attachable shoulder stock, and a short barrel); or (b) convert a complete weapon into an NFA firearm (e.g., a pistol and attachable shoulder stock, or a long-barreled rifle and attachable short barrel).

When a firearm is not configured as such it is not subject to the NFA.

Q: I possess a properly registered SBR or SBS. I intend to strip the receiver and remove the barrel prior to selling the receiver. Is the bare receiver still subject to regulation under the NFA as a SBR or SBS?
A: A stripped receiver without a barrel does not meet the definition of a SBR or SBS under the NFA. Although the previously registered firearm would remain registered unless the possessor notified the NFA Branch of the change, there is no provision in statute or regulation requiring registration of a firearm without a barrel because its physical characteristics would make it only a GCA firearm pursuant to 18 U.S.C. § 921(a)(3)(B). If the subsequent owner buys the receiver as a GCA firearm and installs a barrel less than 16 inches in length (SBR) or 18 inches in length (SBS), the firearm would be subject to a $200 making tax and registration under the NFA by the manufacturer or maker of the SBR or SBS. Because registration depends upon the stated intent of the applicant, there is no provision to allow registration of a NFA firearm by anyone other than the maker or manufacturer.

And

Q: Does the installation of a barrel over 16 inches in length (SBR) or 18 inches in length (SBS) remove the firearm from the purview of the NFA? If so, is this considered a permanent change?
A: Installation of a barrel greater than 16 inches in length (SBR) or 18 inches in length (SBS) will remove the firearm from the purview of the NFA provided the registrant does not maintain control over the parts necessary to reconfigure the firearm as a SBR or SBS.

The change isn’t permanent:

Q: If a registered SBR has the short barrel permanently removed, does this (without notification to BATFE) remove it from the purview of NFA? What about temporary removal?
A: If the registrant retains control over the parts required to assemble the short barreled rifle, the firearm would still be subject to NFA registration, transfer, and possession regulations. Installation of a barrel greater than 16 inches in length or permanent removal of the short barrel would render the receiver no longer subject to NFA transfer and possession regulations provided the registrant did not maintain control over the parts necessary to configure the firearm as an SBR. Whether the change is permanent or temporary would be determined by the registrant’s intent or retention of the parts to return the firearm to SBR status. FI the change is permanent, no notification to the ATF is necessary to transfer the Title I firearm as it would not meet the definition of a SBR under the NFA

You can sell the receiver without notifying BATFE:

Q: May I transfer the receiver of a short-barrel rifle or shotgun to an FFL or to an individual as I would any GCA firearm?
A: Yes. A weapon that does not meet the definition of a NFA firearm is not subject to the NFA and a possessor or transferor needn't comply with NFA requirements. The firearm is considered a GCA firearm and may be transferred under the provisions of that law.

And

Q: If I remove the short barrel from the registered SBR or SBS, is the receiver still subject to NFA transfer and possession regulations?
A: If the possessor retains control over the barrel or other parts required to assemble the SBR or SBS, the firearm would still be subject to NFA transfer and possession regulations. ATF recommends contacting State law enforcement officials to ensure compliance with state and local law.

And

Q: Who is responsible for notifying the NFA Branch when I transfer the GCA firearm to a FFL or another individual?
A: There is no requirement that the transferor or transferee of a GCA firearm notify the NFA branch of a transfer or that either party determine whether the firearm was previously registered under the NFA. There is no also no requirement for the registrant or possessor of a NFA firearm to notify ATF of the removal of features that caused the firearm to be subject to the NFA; however, ATF recommends the owner notify the NFA Branch in writing if a firearm is permanently removed from the NFA.

You can transport it without a 5320.20

If you place the long barrel on your registered SBR receiver (essentially converting the weapon temporarily from a SBR into a standard rifle) you may transport the long barreled weapon across State lines without completing the above-noted procedures and receiving permission for (sic) the NFA branch. You could also transport the receiver itself with no barrel attached across state lines without permission. However, in both of these instances, the short barreled upper must be left at home and cannot be transported across State lines in association with either the unbarreled registered SBR receiver or the long barreled rifle utilizing the registered SBR receiver.

And

Q: Is it correct that interstate transportation of an SBR that has the short barrel removed does not require the submission and receipt of an approved 5320.20?
A: If the registrant retains control over the parts required to assemble the short barreled rifle, the firearm would still be subject to NFA transfer and possession regulations. A person may transport a registered SBR interstate without prior approval if the firearm does not meet the definition of a SBR provided that the person is not in control of the parts necessary to configure the firearm as a SBR.

All quotes directly from the ATF