Bar-Sto makes a drop in barrel for the P229 to change 40/.357Sig guns to 9mm for $200. Their website says some fitting may be required, but everyone on the SigForum sez they have all dropped in fine. No need to change extractors with the Bar-Sto also. While Sig sells replacement 9mm barrels for the P229, it WILL NOT convert a 40/.357Sig to 9mm. Get the Bar-Sto and you will be fine
For aSig P229 .40 to 9mm conversion all you really need is the Bar-sto 9mm converstion barrel and a P229 9mm recoil spring.
You will also need 9mm magazines.
You will also need 9mm magazines.