The 'feds' really don't don't give 2 farts once you're done with the 4473. Outside your BS states, you can pretty much do whatever you want with a lower, or legal gun for that matter. Unfortunately, SBR's (now we have 'pistols') and suppressors are regulated, but other than that, no one cares.
Yes with the one exception of you cannot transition a rifle to a pistol, at least by ATF interpretation. Doing so creates an SBR.
In a free state this never comes up with a stripped lower because we buy them, do what we wish, and nobody ever records if it was made into a rifle (unless some day it is sold as such via an FFL, but then the buyer at least understands they bought a rifle)..
In a nazi state where you record what you've got, if a stripped lower has its serial number pinned down as a rifle, it's permanently a rifle no matter where that lower goes next, with a database in MA documenting it. Now of course if you bought such a stripped lower, made it a pistol, then somehow got jammed up, I would guess claiming whatever record existed in MA was fraudulent might be reasonable enough doubt to eventually set you free - but the fed law does matter here in regard to turning rifles into pistols.