Comment 28 for bug 1671422

As for the charm migration path we need to provide the means for enabling a administrator to morph a existing model without the `cinder-ceph` subordinate to a model with the `cinder-ceph` subordinate.

What I would propose we do is to document the existence and use of the `rename-volume-host` action that was introduced on the `cinder` charm in bug 1665272.

In addition to that we would need to either have the proposed Nova fallback fix landed or find another way to have Nova update the block_device_mapping.connection_info of existing instances with the correct ceph username and the libvirt secret_uuid on the `cinder-ceph` - `nova-compute` `ceph-access` relation.