Is your DNS-323 still functioning? If so, I presume you're not on the market to purchase new HDDs for the DNS-325, otherwise you could simply copy your data from the DNS-323 to DNS-325 via LAN. Unfortunately, as the prior poster described, there is no way to migrate the existing array from the DNS-323 to DNS-325.
From what you describe, it sounds like you may not have a backup of your data. RAID-1 provides redundancy, but is not a backup.
DNS-325 - Data Backup vs. Redundancy. If you don't have a backup of your data, this may be a good opportunity to use the DNS-323 as a backup location (presuming the device still works).