If I'm reading this right:
You have a .223/5.56 Pistol.
1. You would like to register it as a SBR and put a stock on it.
2. You would like to get a second upper and put a SBR 9mm Carbine together too.
If my guess is RIGHT, You WILL need a new tube with a stock +/- spring. Additionally, you *may* need a second buffer +/- buffer spring for the 9mm. You may need a "hookless" hammer for the 9mm as well.
The AR pistol length gas systems put a lot of pressure into the gas tube and therefore push on the bolt carrier group rather violently. They use a pretty heavy buffer to combat the elevated pressures from the short gas tube.
The AR 9mm uppers are blow back. They may work with the heavy buffer and spring, but may not. Be sure that your hammer is correct for the bolt on the 9mm upper. If they are RAMPED BOLT CARRIERS, then you can still use the "traditional" AR hammer. If it is NOT ramped, then you need the 9mm hammer without the "hook" on it. The hook will cause "early cocking" of the hammer as well as put a lot of stress on the hammer pin at least. In the worst case, it can bind up between the bolt carrier and hammer, and that can be a BIG headache. (When the bolt is bound up partially open you can't remove the upper from the lower.)
EDIT: The need to intechage all these parts each time I want to switch calibers really pushed me towards a dedicated 9mm Carbine build. FWIW.