ZFS: Redistribute zvol over all disks in the zpool?
Solution 1:
You would need to rewrite your data to the expanded zpool in order to rebalance it. Otherwise, as time passes, your writes will be distributed across the entire pool.
Solution 2:
There is no reason for the zvol to be stored on the initial devices only. If you enlarge the pool, ZFS will span the updated data on all of the available underlying devices. There is no fixed partitioning with ZFS.