It depends on where you start.
If you buy an AR that has ever been built into a complete rifle, then you can NOT convert it into a pistol.
However, if you buy an AR Pistol you are okay according to ATF Opinion 2011-4, you can convert it into a legal rifle and back into a pistol without fear of violating NFA rules.
Similarly for building one yourself. Build it into a pistol FIRST, then you can go ahead and convert it into a rifle later, and back, without worries of violating the NFA.