I believe, the new unit's firmware will not know the drives are supposed to be in a RAID 1 configuration,
so you can't just drop the old drive into the new unit and get RAID1.
I think you would have to:
1) backup the data
2) setup the old drives as RAID1 (reformat) in the new unit
3) restore the data from backup.
What was the firmware version the drives were formatted with in the old unit?