I recently started using proxmox to run a few VMs and one issue I quickly ran into was wanting to change the ID of a VM I had already created. After a bit of googling I found a solution however it did not work for me because it assumes you are using LVM as your root storage. In my case, I love ZFS so that's what I run on most of my systems. Like docker with containers, proxmox will store VM disks as separate ZFS datasets. With a slight tweak to his code, you can easily change the ID of your VMs when you're using ZFS.
I've written a little script you can put on your proxmox server and after you make sure your VM is not running, a quick call will change the VM id: ./change-vm-id 103 199. The basic process is: rename ZFS datasets for that VM, change the VM disk IDs in the conf file, and then rename the conf file. Once you do this, it should show up in the proxmox GUI with the new ID right away.
#!/bin/bash
POOL=rpool
if [[ $# != 2 ]]; then
cat <<-END >&2
usage: $0 old-id new-id
END
exit 1
fi
old_id=$1
new_id=$2
if ! disks=$(zfs list -r -o name $POOL/data | grep "vm-${old_id}-disk"); then
echo "did not find any disks, check old vm id and running zfs" >&2
exit 1
fi
for disk in $disks; do
new_disk=$(echo $disk | sed "s/vm-${old_id}-disk/vm-${new_id}-disk/g")
zfs rename $disk $new_disk
done
sed -i "s/vm-${old_id}-disk/vm-${new_id}-disk/g" /etc/pve/qemu-server/${old_id}.conf
mv /etc/pve/qemu-server/${old_id}.conf /etc/pve/qemu-server/${new_id}.conf