So I backed up all of my data on the phone (using a combination of Titanium Backup, SMS Backup+, and LifeSaver), and downloaded and flashed the firmware the new firmware using Odin.
Lo and behold, a (very slow) reboot later, pretty much everything worked. All my apps and settings are still there, which is nice.
* Reinstall a custom recovery (TWRP)
* Deleted startup sound (/system/media/audio/ui/PowerOn.ogg)
ADWLauncherEx is quite laggy; so perhaps it needs updating for the latest Android. For now I’m using the updated TouchWiz (the Samsung launcher), but it’s still limited to a ridiculous 4×4 icons per screen. Ridiculous, at least, on the Note 2 which has a very nice large display; might make more sense on a smaller phone. Should be configurable though.
None yet, really. Will need to use it for a few days to see whether there’s any real differences.. So far most of the advertised differences aren’t visible, such as the camera button on lock screen, nor does it feel significantly faster than before.