National Firearms Act Branch
Bureau of Alcohol, Tobacco, Firearms, and Explosives
244 Needy Road, Suite 1250
Martinsburg, WV 25405
(304) 616-4500 (main number)
(304) 616-4501 (fax)
I would just ask them to amend the registry to reflect that you occasionally have the gun in a different configuration than what is hown on your Form 1, and then list those barrel lengths/calibers. Include two copies and a SASE and ask that they stamp one copy “received” and return it to you.
I wouldn’t bother with it, personally, unless I was going to get rid of the original (listed on the Form 1) upper permanently.