Random notes from a security-aware software engineer, open-source advocate and occasional lecturer.
Updating the IP address of a Proxmox server requires more then just updating the network configuration.
Update the network configuration in /etc/network/interfaces by configuring
the new the address, netmask, and gateway for the bridge (typically vmbr0).
Update the host file /etc/hosts which Proxmox uses for internal node
resolution if run as a cluster.
Update the DNS configuration in /etc/resolv.conf if you moved to a
different network and the nameserver has changed.
Update the cluster consideration in /etc/pve/corosync.conf if the node is
part of a corosync cluster. This file is read-only unless you increment the
config_version. So, update the ringX_addr for the specific node and
increment config_version.
Reboot the node.
Note: If you use Ceph, you will also need to update /etc/pve/ceph.conf.