We've been able to change disk storage settings without affecting a container without issue. However other settings should first be tried AFTER you backup the container. I don't expect changing the number of CPUs, etc. will be a problem but backup first. The easiest way I've done this before is to backup the OpenVZ container using vzdump or migrate the container to a 2nd PVE server. Then shutdown the container, and change the settings. Then try and start it again. If it won't start, go to CLI and do it at the command line (Google for OpenVZ commands to do this, or on the Proxmox site). If there is a problem restarting the container, you'll more likely know exactly what it is by reading response back from CLI than trying to do this in the PM web interface. Remember, all PM provides is a simplified web UI to the existing tools that are part of OpenVZ and KVM anyway. Its often better to be able to manage them at the CLI level in case you get into trouble.