On US units this -can- be done, but is not supported by US tech support, and if it breaks it completely is not likely to be covered by warranty.
Make the device RAID 1 between the two drives.
Break the RAID by booting with Drive 2 only. This simulates a hardware failure.
Rebuild to Drive one using one of your new drives.
Break the RAID by booting with Drive 1 only.
Rebuild to Drive two using the other new drive.
Delete the Array in firmware, and rebuild it using the capacity of the entire drive set.
Note that this process doesn't put your data on the new device, takes a very long time, and is unsupported. I'd just go
buy a 421DE.