Yes you can have multiple uppers for one stamped lower. You must be able to readily configure it to what is on the NFA paperwork.
First sentence is correct, second sentence is just another myth on the internet. No such requirement exists.
My NFA attorney says that as long as I can readily change it back I don't need to file any paperwork, but if I'm making the change permanent I do. IE my SBR is registered at a 10" 5.56, I also have a 7" .300 BO. I can swap back and forth all day, but if I sell the 5.56 upper I need to notify ATF of the change in barrel length and caliber.
As Cleric has said it clearly states on the form: "Change of Description or Address:
The registrant shall notify the NFA Division, Bureau of Alcohol, Tobacco, Firearms and Explosives, 244 Needy Road, Martinsburg, WV
25405, in writing, of any change to the description of the firearm in item 4, or any change to the address of the registrant."
That is in 3 places on the Form 1.
You can read the Form 1 here: https://www.atf.gov/firearms/docs/form/f...-53201/download
What source do you have to prove that this is not true?