Actually, no.
Per BATF a lower can be built, disassembled, rebuilt, etc... into a rifle or a pistol, as long as the constructed firearm is legal.
Per the attached 2011 interpretation by ATF, as long as you don't construct an illegal weapon with the parts, you can go back and forth between a pistol and rifle as you wish. It's a long and winding explanation, but worth the read:
The other thing to watch for is constructive intent. In other words, if you have parts that can only go together to build something illegal, don't have the parts. However, if there is any other legal construction that the parts can be used for, you're good to go.
The way I see that is, if I had an assembled AR pistol, and the only barrel I owned was less than 16", and also owned a stock, that would be an issue. I can't build anything with that stock and the parts I have that's not unregulated.
But if I had a 16" or longer barrel, another lower, or even another constructed AR carbine or rifle, I'd be OK. In this case, the stock has another, legal, use. Even if I only had one lower, but had a legal barrel and the stock, I'd be OK. In that case I could disassemble the AR pistol, and then assemble a legal rifle.
This is good news, and by a Holder appointee no less! Here's a copy of the ruling, been this way since 2011:
https://www.atf.gov/files/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf