How to edit or create roles in Proxmox VE

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.

Is your storage not set to allow image files? Go to Datacenter -> Storage tab, select your storage and edit. Under "Content", make sure that "Images" is selected.

I just checked mine and am seeing the same thing as the OP. I would scp the file up and then manually edit the VM's .conf file.

Local storage on proxmox is in /var/lib/vz. There should be an "images" subdirectory with a directory there for each VM (named by the VM number). You can scp the files directly there.

For adding the existing file to the VM, I've had good luck editing the VM's .conf file directly. Look in /etc/pve/qemu-server/ for a file with the VM number followed by .conf.

It's a good idea to create a second test VM so you can refer to it's .conf file to make sure you get the syntax right.

-- adding text from a comment below

I think you can scp the file up. You probably want to look at /var/lib/vz/images/{VMID}/ for the destination. Then maybe look at /etc/pve/qemu-server/{VMID}.conf and add a line for the storage.