How do I fix a capacity problem?

I have a ReadyNAS Duo RND2000 v2 connected to my home network (Windows 7 & 8 PCs).
Until recently it looked like this:
Slot1 - 1TB
Slot2 - 2TB

Drives in Redundant X-RAID configuration.

I started to get messages warning me I had used around 90% of storage capacity. Raidar was showing 841GB of 925GB was used.

I replaced the drive in Slot 1 with a 2TB drive, so the new configuration shows:
Slot1 - 2TB
Slot2 - 2TB

Raidar now shows 93% of 925GB used (ie my capacity hasn't increased).

Checking some other threads on this forum suggests I'm not able to make use of the new 2TB drive because it was limited by the earlier 1TB drive.

I use the ReadyNAS for regular backups from network PCs, and as mass storage for media (mostly photos), most of which is only stored on the ReadyNAS (ie it is not a backup).

So the question is what to do now. How do I get myself into a situation in which I have 2x 2TB drives in Redundant X-RAID congiguration?

I was thinking along the following lines:

• Remove drive from Slot2 (keep safe).
• Format drive in Slot1 (losing all the data but reverting to a 2TB configuration).
• Replace drive in Slot2 and copy all data from it to Slot1.
• Format drive in Slot2 (reverting it to 2TB).
• Revert ReadyNAS to X-RAID configuration.

Does this sound like a sensible thing to do, or should I adopt a different approach?

Note:
I still have access to the previously removed 1TB drive (ie I kept it).
I don't have another ReadyNAS or further networked mass storage for temporary use.

Any help greatly appreciated.