Makes no difference what it says on the lower. It can be marked 5.56 and you can put a 7.62x39 upper on it if you want. Nothing illegal about it. Again if it starts out life as a pistol lower it can be swapped to rifle and back at will. If it starts out life as a rifle lower it must always remain a rifle lower.
This. The caliber marking is just that, a marking. What matters is what it was listed as by the manufacturer when it was first transferred. "Other" or "Pistol", it can be a pistol. "Rifle" and it's always a rifle.