You can also email the atf and state all the information, I emailed my form 4 and Microsoft word document sheet stating what I was doing and got a reply by mail and email in 3 days. Nfamailbox@atf.gov and nfa@atf.gov
This topic was also discussed on another forum. The OP in that discussion stated that they just faxed a letter to the NFA branch and got a reply letter in just three weeks.
This is what I have done. Next owner has ever re-sbrd it with no problems on the exact same serial. Gotta remember a sbr isn’t like a machine gun, the stamp is just basically a permission slip to utilize that particular gun in a sbr configuration. Not like a machine gun where once a machine gun always a machine gun