The easiest thing to do is to create a new Enterprise Vault 'partition' for the Vault Store which contains the existing journal archive. The new partition can be on another SAN, or a completely different device, such as an EMC Centera. You would then close the old partition. All new data would go to the new partition.
You might also consider creating a new journal archive in a different Vault Store altogether. This new Vault Store could be created with a partition on a new storage device. Once you do that, you would change the journaling target to point to the newly created journal archive.
Dave