Now the mechanics of the move are a little unclear to me. Presumably we hook up the 9GB shelf to toaster A and 'raid fail' each 4GB disk in the shelf we want to move? But there will be 5 9GB data disks, since we have to burn one as a hot spare and one becomes the new parity disk. Will there be a problem with having 7 disks to move data from, but only 5 disks to move data to? There would be enough GBs on the 5 9GB disks, but...
Yes, it won't work as you suspect (unless, of course, you do a backup-restore cycle) because the total number of disks in a NetApp RAID group can't be reduced, even if the capacity is increased.
It also won't work because you can't increase the size of a disk once it's on the RAID group.
You can get back your 4GB spare, since you only need one spare. The spare only has to be the same size of the largest data disk in the system. If you have a 9GB spare and a 4GB disk fails, you end up adding 5 GB to the filesystem when the spare replaces the failed disk.
No, you end up putting 4GB on a 9GB drive and getting no more space in the file system than you had before. (If there are multiple spares in the system, the smallest one which is large enough to replace the failed drive will be used.)
-- Karl