I found this on the subject
Once a Rifle, Always a Rifle
The BATFE has made it very clear that once a receiver is built into a rifle, whether by a factory or by you, it is always a rifle and converting it to a pistol is illegal. However, it is perfectly legal to build the receiver into a pistol first and then convert it to a rifle. If the receiver is built into a pistol first, you can configure it back and forth from handgun to rifle and back again as much as you want. However, you must be vigilant to ensure that a shorter than 16″ barrel is never on the receiver at the same time as a stock.
The proper order for converting an AR pistol to a rifle is to remove the short barrel (usually done by removing the entire upper receiver group), installed the 16″+ barrel (usually done by installing an entire upper receiver group), and then installing the stock. I repeat, the stock and short barrel must never be on the receiver at the same time or you will have created an illegal short barreled rifle (SBR).
The proper order for converting a rifle BACK
to an AR pistol is to remove the stock, remove the 16″+ barrel, and finally install the <16″ barrel.
There is some wisdom in building every AR-15 lower receiver that you purchase into a pistol first.
That way you have the maximum versatility should you want to convert it to a pistol later.
Cite: BATFE on converting pistols to rifles