I think what you are referring to is trying to keep the data on the same slots after the conversion is done. If you don't particularly care which drive has which data when you are done, all you need to do is make sure all the data on your largest drive is copied elsewhere, then format it to XFS, and copy the data back to it. When you finish copying the contents of the 4 and 3 back to the 8, you format them to XFS, copy another 2 3TB drives to the now XFS 4 and 3, format those 2, copy the last 3, format, done.
I really, really recommend copying instead of moving, for a couple reasons. First, when you copy, you can verify before erasing the source. Second, it's WAY faster to format a ReiserFS drive which erases it anyway vs. all the deletions that are needed during a move with parity enabled. ReiserFS is slow about deletions anyway, when you compound that with an array write to the destination drive, it can take many hours more to accomplish the same end goal.
rsync -arv /mnt/disk3/ /mnt/disk1
rsync -narcv /mnt/disk3/ /mnt/disk1
The first command would copy all the files from disk3 to disk1, the second command would verify with checksums that the copy is complete. If the second command lists any files, it's because they don't match, and you need to figure out why. One reason would be that you forgot to keep something from writing to the server.