Follow the upgrade documentation as if you are doing new hardware. At the correct point, remove the old drives (label what slot they came out of), put the new drives in and start installing 4.3.
You can not extend / reduce RAID sets, or any of your other theories. You can only do that fancy RAID stuff in storage solutions like Netapp. Which CallManager doesn't support. Well, it might work, but don't think TAC would support it.