finally got around to backing up the 320 data, simply popped one of the drives into my popcorn 410, and started copying to an external drive, only took 5 days!.
then popped both the 320 drives into the new 320L
logged onto the web interface and was immediately presented with a message saying something along the lines of
found a roaming raid from another share centre, would you like to integrate it into your current drive setup. to which I said yes.
and magically all the data is there fine!
can say I am a little surprised it was that easy. least I know have a decent backup of what I started with, along with the NAS being back up again.