I use a Windows XP OS and just went through the up-grade process recently. I too was a bit concerned about what to do with actions, custom shortcuts and plug-ins. I don't have that many of them so I went for the safest approach possible which was to simply re-install them. I did not dump CS. It is still sitting on my computer. You can install CS2 and leave CS alone if you wish. No harm done. (I'll probably dump it shortly, but retain any shared files when askedby the process.) I found I had to re-make custom keyboard shortcuts because CS2 "occupies" certain key combinations that used to be available for custom purposes. I took advantage of the up-grade also to improve on my actions, by simply re-scripting them. Because PK Sharpener Pro is an up-grade, I downloaded the new version directly to CS2 as if it were a new install. Other plug-ins that go to the filter menu I have stored in a separate folder for downloaded applications. I just installed them from that location into the appropriate CS2 filters folder. All works fine.
I did however find a couple of minor "buggy" things with CS2, one has a fix the other doesn't:
(1) In 16 bit, when you add adjustment layers the document size information in the lower right info pane does not change. Adobe is now aware of this. There is no fix yet.
(2) When you use CTRL+0 to fill the screen with the image, it won't do so unless you have first (and one time only unless you change it) clicked on the zoom tool and in the tool's options bar, enabled "Resize Windows to Fit" and "Ignore Palettes". (I'm not sure if this is a generic issue, or my issue because I use a dual-monitor setup for keeping a bunch of palettes displayed on one monitor and the image on the other - anyhow, if you experience that issue, this is the fix that worked for me.)