update from redhat RHEL4 to CentOS5.5

I have a virtual machine running RHEL4 on our cluster server. But IT does not support it anymore. So, I tried to update it to CentOS to enjoy update.
The problem is I have no physical access to the VM, so need to update online.
Neither Redhat nor CentOS suggest to do online update between major releases, but no choice. I did in 2 steps:

1. migrate from RHEL4 to CentOS4
2. update from CentOS4 to CentOS5

Step1 works like charm, but the 2nd step is nightmare. I followed this article: http://planet.admon.org/howto/update-centos-4-to-centos-5-remotely/ because it is the longest one I found. :D I guess it is more comprehensive.

all the steps before 'yum upgrade' is OK, but yum upgrade tried to dump mega error messages (e.g., matchpathcon /.../... failed invalid argument) to my remote terminal, and lasted for more than 10mins, util I pressed ctl+c. Then it left a messy system to me.

What I tried to fix the issue:

1. package-cleanup --leaves (then manually yum remove the packages)
2. package-cleanup --cleandupes
3. yum upgrade (run several times with removing of conflict reported)

Now, the system seems normal.