This was my experience.
I had a MPX I've form 1. Trust name is the make model and serial no. I never engraved the city and sttate. never got around to it. So sue me. Tax was paid.
Fast forward to Nov last year, kinda fed up with it. Wanted to sell it, figuring that selling as a SBR would be a pain in the ass and take forever.
Having heard that all I need to do is revert back to pistol, maybe take it off registry, I decided to call ATF NFA line and ask. The woman I spoke to was helpful. She said as long as I physically had done so, I could sell as a pistol but the info will still be in NFA registry until I request to remove it. Which could be an issue if later transferred or was stolen, recovered, etc. She gave me the info via email and I sent ATF the request to remove it from NFA registry via email and snail mail. I attached the print outs of my request and a copy of the letter I sent to my Form 1.
I took off the stock and sold it.
Just yesterday, June 18, 2020, I finally received a copy showing it was removed from registry, date stamped April 21st. Still took 7 months for confirmation.
My experience with ATF, other LE, gun owners, supposed NFA gurus, we are not all on the same page. For the simple stuff, yeah. For stuff like this... a grey zone.
Remember for crim charges to come your way it will 1st have to be approved by US attorneys office via the ATF. Setting aside the ATF and NFA hate, If someone (gun store) calls the ATF, ATF has the duty to respond. If something looks fucky the gun store has a responsibility to call the ATF and ask clarifications.
For those saying no need to notify ATF if all you do is physically change it back, I say you are mostly mistaken. Physically change it, notify the ATF, sell. Whether you wait the 6 months for a letter back confirming is up to you. Once you've done A and B, and have such proof (email print out, mail receipts) you've prett much met your obligations. And I've also scoured over that NFA manual for this information. Still had to call.