I know of course id have to have 9mm mags.Glocks were great about that. would be great if the Sig could do it as well.
Yes, I have a P226 in 40 and I have all three barrels!
As long as your 229 is a 40 or 357SIG to start with. You'll have to get a Bar-Sto 9mm barrel and may have to have it "fitted".
You might want to change the re-coil spring, a little lighter.