This topic has been beaten to death, and some agents don't even know the law. I do have a copy of a letter received from BATFE which states this answer to that question - Once a rifle it must remain a rifle, regardless of whether it was a legal pistol at any time.

Fox Ridge used to sell a "Carbine Kit" which they claimed was exempt from the ruling. And, I have heard from others that it is okay to go back and forth as long as it started as a pistol. All I know for sure is what I saw in black and white, and that said NO!