doubs43 - from doing quite of bit of research on the p229 and 9mm, .40, and .357sig, it seems that all who have tried going the other way, that is,... using .40/.357sig mags for 9mm, ...have not had any problem. But as was said earlier, you may not be able to go with the 9mm mags for the .40/.357sig
I have a p229r with .40 and .357sig barrels, and I just received a 9mm Bar-sto conversion barrel, which did drop right in. While I haven't shot the 9mm barrel yet, I have put 14 9mm rounds in my 12 round .40/.357sig magazines. The "proof" will be in the shooting, but I am confident (based upon the experience of others) that these mags will work just fine for my 9mm rounds.
I did get the 9mm barrel for cheap(er) target practice. The .40/.357sig is for self defense!
Note - - I will assume you are not considering upgrading a 9mm p229 to .40cal?? I believe that is a no-no, and you probably know that already!