Who -

I am a long time Glock owner and I have successfully switched out upper assemblies several times on various models. Most of the time all you'll need to buy along with the new upper are caliber specific magazines (except for the .40 S&W/357 SIG switch, they are 99% identical). At worst, you may need a new ejector in the lower receiver if the fired brass ejects weakly. I've used a Glock 20 (10mm) upper on a Glock 21 (.45 ACP) frame with 100% reliability as well as a Glock 19 (9mm) upper on a Glock 32 (357 SIG) frame with similar results. All I did was buy the proper caliber magazines for the swap and I was good-to-go.

In your case specifically, the only caution I'll make for changing calibers on Glocks is to be careful moving from 9mm to a more powerful caliber. The original 9mm frames (G17/18/19) were "2-pin" frames made to withstand 9mm recoil only, thus they must remain 9mm only. If you have a newer model 9mm that has the "3-pin" 3rd Generation frame, you should be OK trying a G23 or G32 upper.

Hope that helps some.