I've tested how an upgrade from CentOS 5.2 to SL 5.3RC2 would go. For
this, I setup a CentOS 5.2 system with Apache active. I downloaded the
sl-release-5.3-1.i386.rpm and yum-conf-53-1.SL.noarch.rpm files. When
I tried "rpm -Uvh *" I got the following error: "file /etc/yum.conf
from install of yum-conf-53-1.SL conflicts with file from package
yum-3.2.8-9.el5.centos.2.1". To solve this, I used the additional
"--force" to install both files.

Since there is an issue with glibc (important! see RHEL release notes,
more info here: http://www.pendre.co.uk/screen.php), I updated glibc
first and used "yum update" to do the rest. That went without a
problem. In the end, there were 8 new packages to be installed, 207 to
be updated and 0 to be erased. Total download was 356MB in size.

After the update, I rebooted and then several strange things started to appear:
*Grub*
The splash image of Grub still was the one from CentOS (no update to
the one from SL).

*GDM*
When GDM started, there was a problem with the theme: "There is an
error with the EaseOfBlue theme, can't open file
/usr/share/gdm/themes/EaseOfBlue/EaseOfBlue.xml". Apparently, there is
only one theme in /usr/share/gdm/themes and it's not the right one. It
skipped to an very basic login.

*Gnome panel*
After the login, the icon used on the Gnome application panel still
was the one from CentOS.

Regards,
Patrick