If your RAID array was working properly on your first DNS-325, then you should be able to swap both HDDs into your new DNS-325 without any issue. Before removing the HDDs from your first DNS-325, clearly label the two HDDs as "slot-1" and "slot-2" or "left" and "right", to indicate their original location in the DNS-325 before you make the move. Reversing the HDD order can damage the array.
** If you are using 3TB HDDs, then you'll need to update the DNS-325 firmware to a version that supports 3TB HDDs
BEFORE you install the HDDs. Other than the firmware update, you shouldn't need to install/reinstall any software.
Procedure- Using a magic marker, clearly label the two HDDs in your original DNS-325 as "left" and "right"
- If you are using 3TB HDDs, update the DNS-325 with firmware that supports 3TB HDDs
- Power-down both DNS-325 units
- Move the HDDs from the original DNS-325 to the new DNS-325, making sure to place the HDDs in the same slot number as the original DNS-325