I would suggest that you be on the line for support for this one (but that being said it was easy to do.)
Once the drive is replace and you need to move the data from the Large 450 drive to the 300 drive do this
Run the sysconfig -r
This will give you a list of the drives and the spares will show up in the end
If a spare shows up as NON Zero you must Zero it.
Run this command (it will not hurt anything to run this if there is no disk to Zero)
disk zero spares
Run this command to move the data.
disk replace start (from drive) (to drive)
Example disk replace start 1a.44 1a.17 this will move the data from drive 1a.44 to drive 1a.17
Once the data is move the drive will be available for a spare.