Once again, thanks to all who helped with this particular task.
The link provided by aborzenkov has a note at the end that points out that if you are running DataONTAP >= 8.3, you need to refer to KB 1013046.
As we are running 8.3.2, I followed the procedure described in the KB.
I then had 24 new spare disks and a choice.
I could create 2 new raid groups and allocate them to the existing aggregates (or create new aggregates), or, as the existing rg0 raid groups only had 11 disks each, I could add disks to each raid group and let ONTAP partition them.
This is documented in the Advanced Drive Partitioning FAQ that I got from NetApp.
The advantage of doing this is you don't lose 4 disks to parity roles, you will lose 2 disks to additional spare disks but I can live with that.
Even with the partioning, I still end up with ~ 2TB extra disk space.
Thanks again.