If you ever need to change the hostname of an existing munin node, and you want to retain the existing historical data, follow this procedure:

• Disable or comment out the munin-cron job. On a RHEL or CentOS system, this is typically contained in /etc/cron.d/munin
• Replace the existing hostname entry in munin.conf with the new hostname.
• Rename the existing RRD sub-directory and files.

cd /var/lib/munin
mv oldhost newhost
cd newhost
rename oldhost newhost oldhost*.rrd

• Remove the old HTML directory:

rm -rf /var/www/html/munin/oldhost

• Enable or uncomment out the munin-cron job to restart the service.

Update 2/11/12: Stefan Schlesigner has posted a helpful script for performing this process.