"As easy as buying four same-sized disks all at once" is kinda missing the point.
How do I migrate data from the existing z1 to the z2? And then how can I re-add the disks that were in z1 after I have moved the data? Buy yet another disk and add a z2 vdev with my now 4 disks, I guess. Unless it is possible to format and add them to the new z2?
If the vdevs are not all the same redundancy level am I right that there's no guarantee which level of redundancy any particular file is getting?