Are you running in RAID-1, or some other mode?
I'd do a complete backup of all the stuff on the DNS-323 ( you should have a backup anyway! ) and then stick the new drives in.
You could consider sticking the drives in a USB enclosure and running an EXT2 driver, here's an example: http://sourceforge.net/projects/ext2fsx/
If for some strange reason you're not backing up, I suggest you STRONGLY consider changing your habits!