Please note that I have only tried this on a single Proxmox node, and NOT a node joined to a cluster.
Change to the new hostname in the following 3 files:
nano /etc/hosts
nano /etc/hostname
nano /etc/postfix/main.cf
Reboot Proxmox.
Next, copy the VMs and configuration to the new hostname folder:
cd /etc/pve/nodes/
cp -r old_hostname/qemu-server new_hostname/qemu-server
rm -r old_hostname
cd /var/lib/rrdcached/db/pve2-node/
cp -r old_hostname new_hostname
rm -r old_hostname