You don't migrate the data from the existing z1. It keeps running and stays in use. You add another z1 or z2 to the pool.
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?
This is a problem. You don't know which file ends up on which vdev. If you only use mirror vdevs then you could remove vdevs you no longer want to use and ZFS will transfer the data from them to the remaining vdevs, assuming there's space. As far as I know you can't remove vdevs from pools that have RAIDz vdevs, you can only add vdevs. So if you want to have guaranteed 2-drive failure for every file, then yes, you'd have to create a new pool with RAIDz2, move data to it. Then you could add your existing drives to it in another RAIDz2 vdev.
Removing RAIDz vdevs might become possible in the future. There's already a feature that allows expanding existing RAIDz vdevs but it's fairly new so I'm personally not considering it in my expansion plans.