The factory recommends an 85 bar reg for a 12 fpe Steyr. Some set their regs up to 90 bar. I don't know what your old reg is set to, but 85-90 bar is where it needs to be using the new drop in 12 fpe kit. I would recommend using the new 85 bar reg and keep your old one as a spare. Get someone with a reg tester to check the old one and set it up the same as your new reg.
I have a new 85 bar reg on my Steyr and it's functioning flawlessly.
Hope this helps!