In the NFA handbook, published by ATF, it explains that all NFA guns are NFA by configuration, except for machine guns.
Meaning an AOW, SBR, etc, are only such because they have the features that make them so. If you remove those features, they are no longer regulated by ATF.
Put a 16 inch barrel on a stamped SBR, sell it, no longer an SBR. There is no requirement to notify anyone. You can send a letter to 'remove it from the registry', that they are not obligated to knowledge receipt of, and you might get a response saying 'ok' in 6 months to a year.
If you move interstate with a weapon in the NFA registry, you must notify ATF by law. Having said that, by ATF's own words, put a 16 inch barrel on it, and it's not NFA, so move to a non-sbr state with it (I personally would not take it off the registry). Just make sure you don't have a short barreled upper, unless you have a lawful AR pistol that it goes on.
Thompson Center vs. USA established that if your pile of parts can be assembled into a lawful weapon, that the government cannot assume you are going to make an illegal weapon with it.
Do lots more reading. Be careful who you listen to, as there are a lot of 'experts' that are not.
Don't listen to me, or any other experts. Read, read, read and read some more. Then, do some more reading.