I had lots of fun upgrading an old CentOS box from 3.9 to 4.6.
Faced problems with udev, kernel and openssh 
I will try logging few issues here on my blog which might turn out to be useful for others while attempting the upgrades.
UnixCraft link might give you some inputs for your upgrades. But be careful if you’re upgrading the server from a remote location.
Ensure you won’t remove the font related rpm’s or add the required rpm’s which will allow you to set the correct locale etc. I had to confirm that all these things work before rolling out. I even installed system-language-config to check the language configuration.
udev will get installed fine only if there is a previous version of kernel 2.6 is available on the server. So its best to ask the DC to do the kernel upgrade. I had to force udev installation as I retained the old kernel on the server.
installing the kernel before upgrading the packages might not help you. So, start with CentOs packages, then glib, python, yum, rpm then think of kernel once rest is set fine.
Look forward for more posts soon.
