..." you might be able to convert your 9mm to a .40/.357SIG with an exchange kit. I'm not 100% certain on this, though."
I learned that a Glock 22 /40 S&W ( can I say this in a Sig thread ? ) can be converted to shoot 9mm with a conversion barrel (i.e. Lonewolf)
but a 9mm Glock can't be converted to 40 S&W, as the bolt face is too small, for the wider .40 case.
I'd think the same would be true for Sigs..unless the Sig conversion includes a new slide.
but I have been wrong before.. ( at least once)