I have a very simple Proxmox setup at home. No cluster (anymore), but I did have to convert from 3.x to 4.x to keep current. This means going from OpenVZ to LXC. In reality they are very similar, but it's more or less like changing out the hood on the car (from a sysadmin perspective). The basic theory behind the conversion is not to upgrade the current machine in place, but to do a side-by-side or blue/green deployment in order to retain any information about the old guest VMs and containers as well as keep a back out plan.