You can't go back to pistol on that lower now, anyways. If it has EVER had a buttstock attached, I believe it cannot be converted back to a pistol. You'd have to SBR it. If you buy a stripped lower and put a pistol buffer on it, it's cool - but once you put a buttstock on it that serialized particular lower is never allowed to go back to pistol again without being a registered NFA item - that was my understanding anyways.
So basically you're saying once you go black rifle you can't go back?