I have just upgraded two systems from community script-built Zenoss 4.2.3 to Zenoss 4.2.4 (on a CentOS 6.3 64-bit platform).
I found the documentation a bit scattered and lacking in a couple of places so I have written a wiki item here - http://wiki.zenoss.org/4.2.4_Upgrade .
In particular, the Zenoss Installation Guide says to remove all non-Zenoss ZenPacks. This might be a very destructive suggestion as any device instances whose class relies on a ZenPack, those instances will also be deleted.
I suggest being sure you are well backed up and then attempting upgrades without removing ZenPacks.
I have started another wiki page where we can document ZenPacks that have not been removed for the ugrade an not caused an issue. Please add to this page to make the upgrade process smoother for others. http://wiki.zenoss.org/Zenoss_4.2.4_upgrade_ZenPacks
Cheers,
Jane