If you have a 4-disk RAID5 and you want to convert it to RAID1, you first need to reduce the size of the filesystem on it so it is only using the space of a single device. Then you would convert from a 4-disk RAID5 to a 2-disk RAID5 which requires at least 2.6.30 but I think needs 2.6.32 to be safe. Finally you would convert the 2-disk RAID5 to a 2-disk RAID1 which requires Linux 2.6.33 or later.
I hope that answers your question.
