I believe the command "vol options news root" should be all you need. This operation should not cause any data loss.
Note that the new volume does *not* immediately become the new root volume; that doesn't happen until a reboot.
After it completes copying over the /vol/mail/etc directory to /vol/news/etc, you just need to do a "vol destroy mail" to convert all of mail's disks into spares, and then do a "vol add news <# of spares you want to add>".
The steps should be:
1) copy "/vol/mail/etc" to "/vol/news/etc";
2) do "vol options news root";
3) reboot;
4) destroy the "mail" volume;
5) add the now-spare disks to the "news" volume (leaving, as noted, spares as desired).