[olympus123456]

Are the frames of a 19 and 23 the same? If i bought an entire 23 upper could i use it on my 19 frame?

 

[dglockster]

The 19, 23, and 32 all share the same frame. A 23 upper should work on the 19 lower.

However, you could sell the 19, buy either a 23 or 32 and with just barrel and mag swaps use the same pistol to shoot 9mm, .40, and 357SIG.

 

[jberry]

except...

A G23 with a new barrel shoots 357sig fine

But to get it to shoot 9mm (rather than 10mm) would require a new boltface/ejector assembly - basically a new upper (as well as a new barrel and mag)...

 

[M500]

My GLOCK Armorer said the same thing

the ejector is visibly different from the 19 to the 23, and "might" cause a jam. It is definitely not a factory approved swop to put the G19 barrel and/or slide on the G23 frame, and I will not admit having run three mags of 9mm that way with no hangup. Also, don't try to bend the .40 ejector with visegrips...
The glockfaq.com site is reliable, useful, and in depth.