Comment 1 for bug 1775418

As mentioned in the mailing list, I think this is also something to be controlled in Cinder during retype or volume live migration since that would be a fast fail for this scenario:

http://lists.openstack.org/pipermail/openstack-dev/2018-June/131234.html

Otherwise cinder calls swap volume in nova, which will fail back to cinder, and then cinder has to rollback; it's just easier to fail fast in the cinder API.