I have a sneaky suspicion that as part of the general policy of "work to rule" letter-of-the-law that the NFA Section seems to operate under that because the NFA only authorizes a "short barrel" for an individual rifle, thus "making" an SBR that has three different caliber barrels is the same thing as making three different SBR's, which would likely require one to pay three different taxes and get 3 different approvals, one for each caliber. It is after all the BARREL that defines an SBR, not the receiver. The receiver carries the markings and is the "controlled" part of the gun, but since each barrel of a different caliber in essence creates a different SBR, they view each such combination as a "making" each time the upper is switched, just as putting a short-barreled upper on a non-title II AR lower is "making" an SBR, whereas swapping 16 in + uppers on a Title I isn't regulated at all. I wonder if its even possible to register the same lower for use with different caliber short uppers?
The rules say that you can exchange a 16+ upper for the short-barreled upper and then change them back again without "making" another SBR, and when the registered lower does not have a short upper on it you can use it in your state as a regular Title 1 rifle, but it is still a registered receiver, and if you "maintain dominion and control over" the necessary parts to turn it back into an SBR, then the receiver is still controlled by the NFA, which means that even if you leave your short upper home, you still have to get permission to move the receiver interstate and all other NFA rules apply. If you get rid of the short upper however, the registered lower is no longer governed by the NFA and can be sold or dealt with as any Title 1 firearm can, and there is not even a requirement to notify the NFA Section of the disposition of the registered part, although they highly recommend that you do so.
There's even more complex rules about what you can do if the original base firearm was originally a pistol that's been converted to an SBR. I haven't been able to get a clear understanding of that just yet, though I'm still trying.
Positively byzantine.
It should be simplified: If the rifle is configured with a short barrel, it must be registered and the tax paid and all NFA rules apply. If it's reconfigured with a long barrel, it should be treated like a Title 1 rifle for so long as it is so configured, provided that you don't have a short barrel "in close proximity" to the registered receiver when crossing state lines (you leave the short upper home in the gun safe when you go hunting). This prevents someone from theoretically configuring the rifle as a Title 1 rifle, moving it interstate under those regs while having the short barrel in the trunk and then restoring it to SBR configuration in the new state, thereby weaseling around the notification requirement (which shouldn't exist in the first place, but does.)