Please see comment 5. Thank you!
James, Looks good. Adding one bit of info, Attached volume migration depends on back end support. https://docs.openstack.org/cinder/latest/reference/support-matrix.html#operation_online_extend_support Hope doc mentions/references the need to verify back end support, not just for this new feature.
James, You're correct, my bad. The intent like you suggested was " attached/extend volumes needs back end support."