C'est gruik, très gruik, mais ça a le mérite de fonctionner :

!/bin/bash

RPM="redhat-release-server-6Server-6.3.0.3.el6.x86_64.rpm"
RRPM="redhat-release-server-6Server-6.3.0.3.el6"

function centos () {
mkdir redhat
mv /etc/issue /etc/issue.net /etc/redhat-release /etc/system-release /etc/system-release-cpe redhat/
cp -a centos/* /etc/
rpm -e --justdb $RRPM
}

function redhat () {
mkdir centos
mv /etc/issue /etc/issue.net /etc/redhat-release /etc/system-release /etc/system-release-cpe centos/
rpm2cpio $RPM | cpio -ivdm
cp -a etc/issue.net /etc/
cp -a etc/issue /etc/
cp -a etc/redhat-release /etc/
cp -a etc/system-release /etc/
cp -a etc/system-release-cpe /etc/
rpm -i --justdb $RPM
}

case $1 in
centos) centos ;;
redhat) redhat ;;
*)
echo "missing argument: centos | redhat"
;;
esac