Hi, I think I misunderstood what you said in your original post. I took your words, "move cluster to another subnet" to mean, "we will be changing the IP address of the proxmox nodes".
Broadly speaking my observation with proxmox cluster,
-- it is fussy if you create a proxmox cluster, and then once it is created, you attempt to change either IP or hostname of any cluster member, period. (ie, hence the unsupported 'how to de-christen a cluster member' steps in my post above).
-- the proxmox cluster config is a thing unto itself; and if you 'make a mess of the cluster' - the VMs and their config files which reside on the proxmox nodes are not impacted (lost, damaged, etc) by any mess that might be made of the clusters' operational (or non-operational) status. (ie, ability to manage all cluster members from a single proxmox node WebUI; ability to live-migrate VMs between proxmox nodes, etc - is separate from 'ability to get onto proxmox WebAdmin for <given node> and start or stop a VM running on that node.
-- so as a worst case scenario, even if you render the cluster 'broken' from a 'cluster management perspective' you still have an escape hatch, backup / copy your VM disks and config files to some other safe storage place; blow away the cluster and clean install it with desired configuration in terms of proxmox node IP addresses and hostnames; and then finally restore your VMs back onto proxmox nodes via copying etc.
VMs running on proxmox don't care what the Proxmox management network is / what subnet / etc. So long as they are bridged to proper NICs / with proper connectivity to their required network segment(s). Likewise the proxmox management / cluster does not care about the VMs or their operational state. (other than if you have HA configured and it is 'watching' the VMs to see if they are alive or not / and if it needs to intervene and boot up VMs on another cluster node in the case of a HA-proxmox-node-fail).
if you want to pre-deploy a cluster at one physical location, then power it off, disassemble everything after documenting all the connectivity (ie, what physical port is what subnet / role etc). Then box it all up ship to new location, cable it into service, power it all up, and it works. Since in this scenario you pre-deploy it with the IP / network / hostname config which mimics its final resting place deployment precisely. Or in your case it sounds like you are leaving the gear physically in one location, and wish to cut over proxmox node IPs from one LAN segment to another (different physical or VLAN connectivity to your management interface on proxmox nodes?)
anyhow. If I am still misunderstanding what you are doing, and this is not relevant, sorry for my trouble following what you are trying to do.
Tim