You'll need to create a new pool. If you can hook the new drives up alongside the old drive, then that makes life a little easier, because once you've done that you can then use a tool like "rsync" (or tar, which I prefer) to copy your data over. There are ways to do this that are more clever and ways to do it that are less clever; as long as you don't mind having to do a little reconfiguration, though, "less clever" is probably better. Just add a new pool, copy the data over, and fix your shares and jails to point to the new pool.