Upper Modifications to use with Form 1 lower (SBR)?

As the title states, I was wondering if it’s legal to change barrel lengths on a 16” upper to under 16”, so that said upper may only be used on an SBR lower.

Obviously the upper isn’t considered the actual firearm, nor the NFA item, and I’m aware that changing complete uppers on an SBR is legal, so I don’t see why it wouldn’t be possible to take an upper off a rifle, swap barrels, and place that on an SBR lower.

Thanks everybody!