If your P239 is a 40S&W or 357sig OEM, the barrels are interchangable. If you get a special Bar-Sto 9mm conversion barrel, you can have a 3 barrel gun - my works fine.
A 9mm OEM cannot, however, accept 40S&W or 357sig barrels.
I don't shoot my OEM 357sig barrel much, so if you in the market for a one used about 750 rounds, let me know.
BRET