something sounds wrong here. I still don't think you'd be able to upgrade a 9mm to a .40. Don't get me wrong, I'd love it and would buy one... just seems off a bit.
If I understand correctly the 9mm barrel is thicker to fit the slide. So if the frame is identical, this doesn't seem unreasonable. If the slide were different, there would be no need to have a thicker barrel in 9mm.
Seems like if anyone had (or had access to) both a 9mm and a .40 SC, it would be evident if it would work. Or at least we could know if there were any difference in the frame and slide. Seems pretty easy to take some measurements.
A call to SA would be mandatory, but the question would not be "can I do it?"....the question would have to be "is there a reason why I can't do it, and what is that reason?".
The first question is closed ended. They can just say "no". The second question is open ended and requires an explanation. Certainly if they had a logical explanation, then I would take the "no" seriously. A "no" to the first question gives no information and leave us wondering. And knowing no more than we know without even asking.
would buy a SC if a .357 sig barrel became available
It would seem if one works than the other would as well.
Why can't a tactical or service 9mm go up? (I know it cannot...I just don't know why). Is there a different slide? Different frame? Anyone know exactly what the specific difference is that prevents the barrel switch?