You can Update the Client till some Version, some say not further than 6.4, other say not further than 6.2. Just install the Patches for it and copy the fdb/wdb to your server directory.
Note: You will not have new content, you will just have new stats and items. No new maps, nor instances. For the maps (i'm not sure) you will indeed have new maps, but i guess they will be empty (no luas).
I for myself had my server patched to 6.4, but i do not know if it would have errors in it when you play, since i played not much more than a couple of hours on my server.