ReadyNAS Migration

If the information at the bottom of your message is correct and you have 6 x 2TB drives, then not reallly.  Even if you have RAID6, so 4 of them is enough for a complete volume without redundancy, there is no way to regain that redundancy.  If you have standard RAID5, then you won't even see a valid volume.  You would need a new 6-bay model to migrate the volume.

You can migrate your configuration, just not the data.  You can do that by downloading the configuration file form the old one and uploading to the new.  Or, after backing up all the data, you can destroy the volume and migrate 4 of the 6 drives to the new one (with power off).  Then build a new volume and restore the files.

While more expensive, your best bet is to start the new NAS with new drives.  If 8TB is enough space for now, then start with two of them.  Copy over the configuration, then use backup jobs to copy the data,  You can now use the old NAS as a backup device and you have two spare bays for future expansion.