So one upgrade later and it seems to have gone to plan. I thought I'd install ZenUp and see what's what. Some notes:
1) The install guide just says to install zenup. At the moment you have to download/install via http from the wiki. I know it's theoretically in beta, but it really should be added to the Zenoss Dependancy Repo...
2) ...because I think in order for it to work, it requires the quilt RPM which is in the Zenoss repo.
3) Depending on how you interpret the install guide, you may try and use ZenUp. The catch is that at that point in the guide you haven't installed 4.2.4 yet, so if you followed it through to its natural conclusion then you would have just patched 4.2.3!
4) The final step in using ZenUp is to run zenup diff and it tells you it should show 0 modified files since the patch. However I have written some custom data collector plugins and they then showed up at this point (as they are not part of the pristine file list or the actual .zup file list). That probably should make a note that any customisation involving new files in certain places will be picked up.
5) The layout of the wiki page is a bit all over the place (one of the problems of WYSIWYG HTML editors), so I've cleaned it up a bit, as it is a wiki after all!