RAID6 -> RAID1 can be done, but not as a single operation.
First you convert to RAID5.
Then you shrink that to 2 drives.
Then you convert (instantly) to RAID1
Then if you wanted RAID1 over more than 2 devices, you add the extra devices in.
I could possible teach mdadm to perform these three steps in sequence, but I'm not sure the demand would really justify it...
