I cannot find any Glock caliber change kits, I do find 3rd party barrels that were milled to fit into a Glock slide which is not a Glock caliber conversion. You can custom build a barrel to fit into any handgun slide but that is not the same thing as the gun company selling caliber change kits for their own firearms. Sig sold caliber kits for their P226 and now they sell them for the P320. Glock does not do this.
I know what you mean though but Glock does not support or cover their firearms with a 3rd party barrel in it.
Also shooting 9mm from a 40 without changing out the extractor is going to give you issues at some point and so is not really a conversion.
I have 3 different uppers for my P226 one is for 22lr one is for 9mm and one is for 40sw / 357sig so I have one handgun that shoots 4 different calibers and fully supported to shoot that way by the manufacturer
I agree with the point on Glock not offering conversion kits, but I just want to point out to everyone that, if OEM if what you're after, you can swap a complete G17 slide onto G22 frame and use G17 mags, then all parts are Glock OEM that way, which is essentially similar to what a Sig conversion kit does. At this point, the only thing left that's different is the ejector, which can be replaced but not necessary for the function of 9mm ejection. If you do replace the ejector, then at this point you basically have a stock G17 as all other parts are the same.
Also note in the above scenario, you save money on the frame minus the ejector.
On the other hand, like the other fellow mentioned, there are aftermarket conversion barrels available to keep the cost down for converting .40 to 9mm (or to .357). At this point, you have the option to further the conversion to make it more reliable by replacing the following (to OEM): Mags, Extractor, Ejector. Usually in that sequence with respect to their roles on reliability improvement. Assuming all of them are replaced, then you essentially have an stock G17 with aftermarket barrel, which is what a lot of people already do to their G17's anyway as a customization.
In the above scenario, you save money on the frame minus the ejector and the slide assembly minus the barrel.
In my opinion, in terms of versatility and flexibility, both P320 and Glock got that covered. The P320's modular design is more innovative and likely to be a trend for future pistol designs, but to work on the P320 trigger group is much more complicated than working with the Glock. The Glock has much less parts and it's very easy to work on.