Just wanted to give a quick update for anyone in the same boat as me. First, thanks again to barney & peter (as soon as he came around) for helpful replies.
Anyway, the update...
1. Ugraded to Mavericks.
2. Third party icons (e.g. MSOffice pack, Chrome, etc.) stayed the same as I had them.
3. Apple's apps reverted back to rainbow brite's barf (as expected with update).
4. Third party apps for icon modification (e.g. Candybar) indeed do NOT play well with 10.9 (just doesn't work—nothing damaging).
5. The "get info, paste" method of replacing icons works perfectly fine for most apps. Using this method in Mavericks, icon changes are retained better than they were for me in previous OS X (as stated before, I never rule out the possibility that I was being an idiot the first time around). The only catch is that the resolution for scaled-down icon sizes is not as good as if you switch icons out properlly/fully at resource files.
6. For apps that were a bit trickier (Finder, trashcan, iCal), the same fixes that have always worked continue to work fine (replacing resource files using well-published instructions all over the interwebz).
7. As with aforementioned trickier apps, system folder icons, need to be changed manually. It's a bit of a pain and time consuming, especially if the old icons being used did not have all sizes/resolutions used by 10.9.
8. Now that I've made new versions for quite a few of the apps and system folders, the front-loaded effort will make future changes easier if/when icons revert. So, yeah it kinda sucked, but it'll be immeasurably easier to manually replace next time around.
- Random tip: organize replacement icons into folders corresponding to folders for resource filles and then it's easy to batch-replace a whole set of icons at once (e.g. keep all the CoreTypes.bundle replacement icons in one folder, Dock resources in another folder, etc.)
9. No major hang-ups or glitches so far, and actually more pleased with UI tweaks than previous OS X.
In short: Icon customization is a bit of a pain and a manual process, but I was probably being a bit whinier than needed when dreading worst-possible-scenarios pre-switch, results are great, and glad I made upgrade.
And of course, the obligatory reminder to anyone out there who stumbled across this thread, considering making changes to system/resource files (or OS updates for that matter): backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup, backup... aaaaannnd backup.