Verified on:
openstack-cinder-11.1.0-14.el7ost.noarch

Add NFS backend to existing lvm, add availability zone dc1 dc2 

enabled_backend =  tripleo_iscsi,nfs

[tripleo_iscsi]
volume_driver=cinder.volume.drivers.lvm.LVMVolumeDriver
volumes_dir=/var/lib/cinder/volumes
iscsi_protocol=iscsi
iscsi_ip_address=172.17.3.18
volume_backend_name=tripleo_iscsi
iscsi_helper=lioadm
backend_availability_zone = dc1

[nfs]
volume_backend_name=nfs
volume_driver=cinder.volume.drivers.nfs.NfsDriver
nfs_shares_config=/etc/cinder/nfs_shares.conf
nfs_snapshot_support=True
nas_secure_file_operations=False
nas_secure_file_permissions=False
backend_availability_zone = dc2


echo "10.35.160.111:/export/ins_cinder" >  /etc/cinder/nfs_shares.conf


systemctl restart openstack-cinder-volume.service

Create types:
cinder type-create lvm 
cinder type-create nfs
cinder type-key nfs set volume_backend_name=nfs
cinder type-key lvm set volume_backend_name=tripleo_iscsi
cinder extra-specs-list
  
Create volume on nfs/dc2 
cinder create --display-name nfs-dc2 --volume-type nfs 1 --availability-zone dc2


cinder list
+--------------------------------------+-----------+---------+------+-------------+----------+-------------+
| ID                                   | Status    | Name    | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+-----------+---------+------+-------------+----------+-------------+
| 80f32fb7-b566-40d7-932b-d83eb21b609e | available | nfs-dc2 | 1    | nfs         | false    |             |




Migrate to other zone (nfs to lvm) 
cinder retype 80f32fb7-b566-40d7-932b-d83eb21b609e lvm --migration-policy on-demand

During retype operation

cinder list
+--------------------------------------+-----------+---------+------+-------------+----------+-------------+
| ID                                   | Status    | Name    | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+-----------+---------+------+-------------+----------+-------------+
| 80f32fb7-b566-40d7-932b-d83eb21b609e | retyping  | nfs-dc2 | 1    | nfs         | false    |             |
| b96926aa-6a99-4f11-851f-7d9b9c1fee40 | available | nfs-dc2 | 1    | lvm         | false    |             |


Once operation completed we can see volume was retyped/migrated to other AZ.  

cinder list
+--------------------------------------+-----------+---------+------+-------------+----------+-------------+
| ID                                   | Status    | Name    | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+-----------+---------+------+-------------+----------+-------------+
| 80f32fb7-b566-40d7-932b-d83eb21b609e | available | nfs-dc2 | 1    | lvm         | false    |             |

With cinder show we can see

cinder show 80f32fb7-b566-40d7-932b-d83eb21b609e
+--------------------------------+---------------------------------------+
| Property                       | Value                                 |
+--------------------------------+---------------------------------------+
| attached_servers               | []                                    |
| attachment_ids                 | []                                    |
| availability_zone              | dc1 ----> changed from dc2 to 1       |
..                                |
| id                             | 80f32fb7-b566-40d7-932b-d83eb21b609e  |
..                                      |
| migration_status               | success                               |
..                                |
| name                           | nfs-dc2    
..
| status                         | available       

Works as expected.

Retyped back to NFS
cinder retype 80f32fb7-b566-40d7-932b-d83eb21b609e nfs --migration-policy on-demand

Again with Cinder show we witness that volume moved back to nfs/dc2 and available

cinder show 80f32fb7-b566-40d7-932b-d83eb21b609e
+--------------------------------+--------------------------------------+
| Property                       | Value                                |
+--------------------------------+--------------------------------------+
| attached_servers               | []                                   |
| attachment_ids                 | []                                   |
| availability_zone              | dc2    -> back at dc2                |
                         |
| id                             | 80f32fb7-b566-40d7-932b-d83eb21b609e |
| metadata                       |                                      |
| migration_status               | success                              |
| multiattach                    | False                                |
| name                           | nfs-dc2                              |
| os-vol-host-attr:host          | hostgroup@nfs#nfs                    |
| os-vol-mig-status-attr:migstat | success                              |
| os-vol-mig-status-attr:name_id | None                                 |
                                |
| status                         | available

Retested with a none empty volume worked as well.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:2516