Amendment? I would think if anything is chaged it would necessitate filing a new 4.
No. You can apply to BATF for an amendment to your Form 4 if, for example, you add a calibre to your existing NFA "weapon," be it another upper in a different calibre for a registered lower or an additional new weapon that accepts a registered auto sear (in that case, the sear is the weapon).
I need to know the procedure, but I can't find it anywhere on ATF's website. I think I will do as the previous poster said and call them.