This is a bit exciting for me - less so for anyone else I guess :(
But it has taken me a while to get to this point.
Took a snapshot backup from the office proxmox V3.x server, and brought it home t o convert to LXC (Linux Container) format on a proxmox 4.2 server (apparently LXC is the go nowadays - who knew?)
Here's the magic bit:
root@pve:/var/lib/vz/dump# pct restore 100 -storage local-lvm vzdump-openvz.tar.gz
Logical volume "vm-100-disk-1" created.
mke2fs 1.42.12 (29-Aug-2014)
Creating filesystem with 1835008 4k blocks and 458752 inodes
Filesystem UUID: b297212a-5d45-4ded-9a5a-44d7eaa246f9
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Multiple mount protection is enabled with update interval 5 seconds.
Writing superblocks and filesystem accounting information: done
extracting archive '/var/lib/vz/dump/vzdump-openvz-100-2016_05_17-13_55_00.tar.gz'
Total bytes read: 5954232320 (5.6GiB, 34MiB/s)
Detected container architecture: i386
###########################################################
Converting OpenVZ configuration to LXC.
Please check the configuration and reconfigure the network.
###########################################################
root@pve:/var/lib/vz/dump#