Let's see if we can sort this out. Depending on what I read, I got the impression that the original P229 was chambered in 9mmP or .40Auto.
The P229/9mmP has a smaller OD barrel than that used in the .357SIG or .40Auto. Of course, the breech face on the 9mmP version of the pistol is slightly smaller, too.
If you want a legitimate three-caliber pistol, buy the P229 in either .357SIG or .40Auto configuration; add a factory barrel for roughly $130 for the companion caliber [where mags are adequate for either caliber], then buy the 9mmP barrel from Bar-Sto [which will most likely drop in because it is 99% fitted and the correct OD].
Not to say that going from the P229/9mmP to .357SIG or .40Auto is impossible or inadvisable, but I've never seen such a conversion.
I've seen many of the opposite exchange...