Just arrived home from the Wisconsin State IDPA Championship Match at which I shot my trusty Glock 17.  As luck would have it I won an older style (no finger grooves or access. rail) Glock 22 with night sights.  I still have to wait 2 days to pick it up but I already have a couple question:  Can I put the upper slide assembly from the 22 on my new stlye 17 frame?  Can I plink with .40 ammo out of my pile of 9mm ten round mags (I figure they won't hold ten rounds of .40 though)?  I know some models of Glocks are slide assembly swappable but can different generation models also share slide assemblies?  Thanks!

you need to go to

Glocktalk.com
or
Pistolsmith.com

You'll get a lot better info there.

You can also get a 9mm conversion barrel for the G22 that will work just fine when using your G17 mags.  The conversion barrels should be priced for under a $100 from either Federal Arms (Guns-n-Stuff catalog) or Olympic Arms.  No other additions are needed.

The 9mm conversion is possible, with two important notes.
1) you must buy an aftermarket conversion barrel, not a GLOCK OEM 9mm barrel. The outside diameter of the barrel is smaller than the .40. This is not the case with the .40 and .357, which swap right out.
2) you should buy a 9mm magazine. The .40 mag will work sometimes, but the feed lips are wider, and it can double feed. New mags are less than $20.

It is sometimes alleged you need a new extractor and ejector. I have conversions for my G23 and G35, have not replaced these parts, and have had zero problems. You will know immediately if you need a new ejector, as a small percentage of .40's firing 9mm will throw the brass back at you. If this does not happen immediately you are fine. As to the extractor, some say the slightly different design causes stress, and it may break. As far as I can tell this is hypothetical, there has not been a single documented case.

Mine has been flawless as well (as long as you use a 9mm mag).  Many folks over at GlockTalk have tried the conversion and have reported the same.  My barrel was from Olympic Arms and it has never failed.