I've used FreeNAS and OpenFiler and I preferred OpenFiler. I noticed however on both systems that their performance was piss poor so I moved to Nexenta.
It's been a bitching long time but I'm fairly certain you'll have to re-create the zPool, so you would lose all your data. I would backup your data before you try anything either way. A 3TB USB drive isn't too expensive from amazon.
Side Note: FreeNAS's performance is pretty piss poor when it comes to throughput. I went to Nexenta because it's an enterprise class SAN OS and provides my virtual farm (40-60VMs) a stable, fast and robust environment to operate in.