Westy, in order to construct a federally legal AR pistol, you need a lower receiver that has never been assembled as a rifle receiver. After the manufacturing process for the lower receiver is complete, if ever the receiver is registered or assembled as a rifle receiver, the ATF states that that receiver is forever a rifle receiver. Any attempt to assemble an upper receiver with a barrel length of less than 16 inches is no longer legal without the tax stamp for the lower receiver and entry of the receiver into the national registry of Title II weapons.
All that that convoluted explanation means is that the receiver needs to have been registered by the manufacturer as a pistol receiver and never previously assembled with rifle/carbine stock components or transferred as a rifle receiver. While your at it, it's a really good idea to get a letter from the manufacturer stating that the lower receiver was originally manufactured as a pistol lower receiver and never assembled into a rifle lower receiver with rifle components.
Whereupon you assemble your pistol upper to your bonafide pistol lower, in the eyes of the ATF, the firearm that you've assembled leaps into existence as a pistol.
Afterwards, it is then legal for you to convert the lower to a rifle using rifle components, after you have removed the pistol barreled upper receiver, provided that you also have another pistol lower receiver to assemble your pistol upper receiver with. Having a spare pistol upper separated from the pistol lower that you have assembled into a rifle lower receiver is viewed as "constructive intent to manufacture or possess a short barreled rifle". This is not legal without the tax stamp for a short barreled rifle receiver. Make sure you have the same count of uppers as lowers. In general, if you own AR pistol and AR rifle lower receivers, make sure you keep the pistol uppers with the pistol lowers and rifle uppers with the rifle lowers. Mixing and matching is illegal except in the manner I described.
That's the best I can construct of the intent of the ATF from all the convoluted opinion letters and guidance that they've published pertaining to your question. If any of that is not clear to you, perhaps you ought not possess or assemble your pistol upper receiver to a lower receiver until you have the proper type of lower receiver in your possession.
Basically, know and understand the local, state, and federal laws pertaining to Title I and Title II firearms and cover your rear end.