Depends on what the Form 4 says. If the form states a certain caliber, then any upper in that caliber is OK. The thing to do is list "multi calibers" on the Form 4. Then you are good to go with ANY upper.
The full-auto firearm part with the SN is the registered part. IE: The lower on an AR-15/M-16 with the original auto sear, the DIAS itself when those are used in ARs, and the bolt itself on certain UZIs.
SubGunFan; Look at the 5320.4 on your MAC-10. It lists a barrel length as well as an overall length. Swapping out the OEM upper with the MAX-10 upper obviously changes the barrel length along with the overall length. Same thing if you utilize the OEM upper but replace the wire form stock with another more rigid type of stock. You've changed the OAL listed on your 5320.4 but you're not required to notify the BATF because the change isn't a permanent one. If you weld a fixed stock to the registered lower receiver, you'd have made a permanent change and you need to notify them.
Caliber conversions follow the same guide lines as OAL. As long as the caliber swap or barrel length isn't permanent, you're good to go and there's no need to notify the BATF. If you make a permanent change, you must notify them in writing.
Currently there are a couple of .22 conversion kits shipping for the MAC family of firearms and there are no permanent modifications made to the registered lower receiver. These are a simple one pin drop on/in conversion and there's no need to notify BATF.
Most M-16 owners routinely swap uppers and calibers to include. .22 LR, 9mm, 7.62x39, 5.45x39, 7.62x25, to name but a few. Again, there are no permanent modifications made to the registered receiver so you're GTG.
Full auto title two firearms aren't all that different from title one weapons in this regard. Would you notify BATF if you converted your semi auto AR-15 to shoot 9mm or .22LR for the afternoon? What if you replaced the 16" upper receiver with one that was twenty inches? You wouldn't notify them and you're perfectly fine.
Many beltfed owners are using 7.62x54r or 8mm ammo conversions due to the high cost of 7.62x51, .303, and .30-06. Most consist of a fast and simple bolt/lock, and barrel swap to convert the gun to a cheaper to shoot caliber. BATF isn't being notified.
If you desire, you can write BATF and advise them you'll occasionally shoot your registered machine gun in XXX caliber and replace the listed barrel length with one that measures XX inches. Don't expect a reply or confirmation from BATF.
On many transferable title two weapons as well as semi auto title one guns, the lower receiver is the registered, controlled, or regulated part. Swapping the uncontrolled upper receiver doesn't change the function or the status of the lower receiver.
I'll bet you own at least one caliber conversion for at least one gun. I've got a 9mm conversion barrel for a .40 S&W Glock 22. The lower receiver or frame is the controlled part, not the barrel or upper. Look at the Thompson Contender handgun. One controlled lower receiver and many over the counter barrels available shipped directly to your door.
In many ways, the same laws that govern title one firearms also apply to full auto weapons.