Comment 10 for bug 1396965

After working half of the day on this issue on my local installation, if all you want to do is to detach a root volume, two updates in mysql are sufficient:

use cinder;
update volume_attachment set mountpoint='/dev/vdb' where volume_id = 'oldVolumeId' and deleted = 0;
use nova;
update block_device_mapping set device_name = '/dev/vdb', boot_index=1 where volume_id = 'oldVolumeId' and deleted = 0;

basically the mountpoint/device name must not be /dev/vda and boot_index must not be 0. If this is true, you can successfully remove the volume using
nova volume-detach <instanceId> <volumeId>.

To attach another volume, do the same thing in reverse after having used nova volume-attach. That is, set the name back to /dev/vda and set the boot_index to 0 for the new volume. Basically below:

use cinder;
update volume_attachment set mountpoint='/dev/vda' where volume_id = 'shinyNewVolumeId' and deleted = 0;
use nova;
update block_device_mapping set device_name = '/dev/vda', boot_index=0 where volume_id = 'shinyNewVolumeId' and deleted = 0;

Then again use the magic incantation:

nova reboot --hard <instanceId>

to force a fresh and correct libvirt config for the instance being created.