1.8 iso upgrade

Hello, we have an server which is running Proxmox 1.8. Plan is to Upgrade to 5.1. As we only have Windows VMs i want to use Windows Backups to get baremetal backups. Afterwards server will be wiped, Proxmox 5.1 installed and the VMs will be restored. If something goes wrong a Wayback to 1.8...