Most semis in 40 can easily be converted to .357 SIG with just a new barrel. HK USPC can be converted easily as can the P2K, P2KSK, P30, P30L, but NOT the USP.
NOTE: The .357 SIG is NOT just a necked down .40. Since the .357 SIG actually headspaces on the case mouth instead of the shoulder (like it should have) if you neck down the .40 S&W case the case will be too short to headspace properly on the mouth. So, one cannot simply take a .490 S&W case, run it through the .357 SIG sizing die and come out with a perfectly formed .357 SIG case.
Other than that, a new barrel, some ammo, and you are good to go.
This usually also works when going from a .357 SIG to .40 S&W. All you need is a new barrel, usually.