Find three spare disks, if necessary - remove ownership from them, boot new head into maintenance mode, assign ownership of these disks to new head, boot into special boot menu and select 4 (or 4a, depending on version) to initialize root volume. After reboot you will need to do usual setup of new controller.
Make sure to install the same Data ONTAP (including patch number) as is already used before doing it for 8.x or netboot using the same version if using 7.x. In case of 7.x do not forget to "update" Data ONTAP and download correct kernel to boot device.