How to rename a ZFS Volume on FreeNas
What you basically need to do is export the zpool using the shell command line (not on the web interface).
The commands a fairly simple.
Keep in mind that afterwards, at least I had to do it, to detach the volumes (zpools) from the system (DON’T CHECK THE OPTION TO ERASE DATA).
Auto-import volumes (zpools).
And bingo you have the volumes back with the new names and all of your data kept intact!
More Information [Detailed]:
http://doc.freenas.org/index.php/Volumes [Detach Volume Section]