do the paperwork with the ATF to make it a SBR and pay the $200.00 tax fee Even if you change the stock to a brace it will still be a SBR and need the SBR paperwork (as it was registered as a rifle and has been modified) you can buy a lower and make a pistol even if he changed it to a brace it would still be a SBR as it was made as a Rifle and need the tax stamp and paperwork " Very simply, the pistol AR-15 can NOT have a buttstock of any kind. And it must NEVER have had a buttstock of any kind attached. The actual lower receiver is bought/transferred into your name as a pistol lower, not a rifle lower. (In other words, if you bought an AR-15 rifle with a buttstock and then took the buttstock off, while it may look EXACTLY like an AR-15 pistol — the ATF still considers it a rifle.)" http://preparedgunowners.com/2017/04/27/atf-reverses-opinion-on-ar-15-short-barrels-with-braces-again/

Benjamin, Register it as a SBR Short Barreled Rifle ($200 Tax Stamp and NFA Registration) if you plan to keep the Shoulder Stock. Otherwise, replace the Shoulder Stock with a Brace and change away. I would honestly just buy a pistol lower with brace for two complete firearms and be done with it. Cheers and stay safe!