would the stock need to be replaced by the original brace?
Not a lawyer, but I don't see why there would be any need to replace the stock with a brace.
You have the SBR tax stamp for your firearm. Your gun is now legally registered to be an SBR. ATF doesn't care what parts you put on it, NOW. If it is in a legal, approved
licensed configuration, they can't make you change it, unless they revoke the license, and they need specific cause, to do that. (like you get convicted of a felony..)
Look at how we got here, and be sure to include the information the popular media leaves out or obscures.
Under the NFA 34, braced pistols are not regulated. AT ALL. STOCKED pistols are. Regarding the AR pistols the ATF ruled that if it had a brace, and the item was designed and intended to be a brace (no matter how it was used) then it was a brace, not a stock, and therefore not regulated under the NFA 34.
Then, later, (for various reasons I'm not going to list right now) they changed their minds and the ATF essentially reversed their ruling, and said "what was once a brace is now a stock" and since it is a stock it is an NFA item and you must register it as such.
They were "kind" about it, giving us the options of destroying the gun, surrendering the gun, reconfiguring the gun to full pistol, or registering it and getting the tax stamp. They offered a "sweet deal" with an amnesty period where you could get the stamp for free, and not pay the $200 tax.
The court has put a stay on the requirement. The requirement is still valid at this time, and remains valid until we get a court ruling upholding it, or invalidating it. The stay means we are not required to comply with the ruling, (and the govt cannot prosecute us for non-compliance) until the stay is lifted (or expires).
If you have already complied with the ruling, there is nothing requiring you to undo what you already did. If you put an actul stock on the pistol, then it IS an SBR, and you've got the stamp making it legal. This will not change no matter if the ruling is upheld or vacated.
IF you've got a brace the ATF says is a stock and you've already registered it as such, that won't change, either.
Clear as mud, yet?...