Just reciently I've found my self down to 100GB of my 6.4 TB NAS and decided it was time to upgrade my storage space. Now upgrading storage space on freenas isn't as simple as just replacing a bunch of drives; you must replace them one at a time and you wait for the drive to be completely replaced (resilvered) before replacing another drive otherwise you risk data corruption or loss. By default the auto grow is set to off so if you want to grow the space on your nas you must enable the auto expand feature in freenas. As always you should have another backup of this data.
So on your FreeNAS server you want to login via SSH or use the shell console provided in the web interface. Type the following command.
Once that is enabled you may now grow your freenas storage by replacing drives. As for myself I've replaced 4 320GB drives with 4 1TB drives moving my NAS from 6.4 GB to 8.3GB of storage space and to replace the drives it is a daily process with replacing a drive once a day until they are done.