Please note this tutorial has not been tested, but in theory should work.

The following tutorial provides the few steps to upgrade Red Hat 9 to CentOS 4:

1. Make a full backup of your data.

2. Import GPG keys:
rpm --import http://beta.centos.org/centos/4.0beta/os/i386/RPM-GPG-KEY
rpm --import http://beta.centos.org/centos/4.0beta/os/i386/RPM-GPG-KEY-centos4.0beta

3. Install/Upgrade yum:
rpm -Uvh --nodeps http://beta.centos.org/centos/4.0beta/os/i386/CentOS/RPMS/yum-2.1.13-2.centos4.noarch.rpm

4. Upgrade the release:
rpm -i http://beta.centos.org/centos/4.0beta/os/i386/CentOS/RPMS/centos-release-4.0beta-5.i386.rpm

5. Have yum update your OS:
yum upgrade

The following tutorial provides the few steps to upgrade Red Hat 9 to CentOS 3.4:

1. Make a full backup of all your data

2. Import GPG Keys:
rpm --import http://beta.centos.org/centos/3.4/os/i386/RPM-GPG-KEY-CentOS-3

3. Install/Upgrade yum:
rpm -Uvh --nodeps http://beta.centos.org/centos/3.4/os/i386/RedHat/RPMS/yum-2.0.8-1.centos.7.noarch.rpm
rpm -Uvh --nodeps http://beta.centos.org/centos/3.4/os/i386/RedHat/RPMS/centos-yumconf-1-11.noarch.rpm
rpm -Uvh --nodeps http://beta.centos.org/centos/3.4/os/i386/RedHat/RPMS/centos-yumcache-3.1-0.20050105.3.noarch.rpm

4. Upgrade the release:
rpm -i http://beta.centos.org/centos/3.4/os/i386/RedHat/RPMS/centos-release-3-4.2.i386.rpm

5. Have yum update your OS:
yum upgrade

Similar Articles : Upgrading CentOS 3.3/3.4/3.* to CentOS 4, Upgrading Red Hat 9 to CentOS 3.4 or 4, Upgrading Fedora Core 1 or 2 to CentOS 3.4 or 4