You can manually scp or rsync the disk from one location to another.
First shutdown your lxc container. Then rsync or scp it from one storage location to another - this location should already be mounted in PVE so you don't cause any issues.
You will need to know the name of your node, the vmID - normally 3 numbers.
Then edit the config file on one of your proxmox nodes:
pve/nodes/<node>/lxc/<vmID>.conf
There should be a line that reads:
rootfs: <old-location>:<vmID>/vm-<vmID>-disk.raw,size=XXG
Just change the old location to the new location.