I recently upgraded my homelab Proxmox host server and wanted to repurpose my existing server to use as a testing environment. Below are the steps I used to rename the old Proxmox instance. These steps worked for me. Please let me know in the comments if you try these steps and any issues arise that I didn't have in my environment.
NOTE: Renaming a node that is part of a cluster is not recommended. These steps are intended for standalone nodes. Always make backups of important VMs before attempting to modify the Proxmox host.