Actually you can change the hostname on the netapp without disruption to nfs clients, and I dont think it disrupts CIFS until you establish a new machine name by rejoining the domain/AD. You need to edit rc hosts files then issue the hostname command -- you do not need to reboot the head (or for that matter issue a takeover/giveback) -- will take the new hostname. The disruption can come when you actually update your external DNS servers. There are tricks you can do depending on how your clients handle DNS changes. IE changing DNS so future mounts will take on the new hostname etc.
TO note, you can actually change DNS forward/reverse records, and the netapp itself doesn't need to change its own hostname.