With my 7b5, if i use /centos/4/ in /etc/yum.conf and i run yum update : no problem ??? 
I believe this should work. Please let me know if it doesn't and I'll update these instructions.
WARNING: You will be pulling down around 200Megabytes of updates. You are probably better off upgrading from the CD in most cases.
- Go to the Software Installer panel
- Disable all repositories except for smeos
- Install the update for smeserver-yum (and only that one)
- Enable smeos, centos and centos updates repositories
- Go to the command line and type:
yum update kernel
- Check that the new kernel is the default in /boot/grub/grub.conf
yum update # WARNING: About 180Megabytes
- Go to Shutdown and Reboot panel
- Select "Reconfigure"
- System should run "post-upgrade" and then reboot
Gordon