When we execute neutron port-update, new instance-ip is created and attached to existing VMI.
root@contrail-01:~# neutron port-update 5588a691-9695-4a02-b72d-5338b869f866 --fixed-ips type=dict {ip_address=10.1.0.23,subnet_id=b966187e-bde7-42a9-b15b-3473e68e06dd}
Updated port: 5588a691-9695-4a02-b72d-5338b869f866

root@console:~# ./Documents/python_test/Contrail_json.py -d virtual-machine-interface/5588a691-9695-4a02-b72d-5338b869f866
{
    "virtual-machine-interface": {
        "display_name": "5588a691-9695-4a02-b72d-5338b869f866", 
        "fq_name": [
            "default-domain", 
            "unregard1", 
            "5588a691-9695-4a02-b72d-5338b869f866"
        ], 
        "href": "http://172.27.113.202:8082/virtual-machine-interface/5588a691-9695-4a02-b72d-5338b869f866", 
        "id_perms": {
            "created": "2015-12-17T04:05:18.153750", 
            "creator": null, 
            "description": null, 
            "enable": true, 
            "last_modified": "2015-12-21T05:27:03.367422", 
            "permissions": {
                "group": "admin", 
                "group_access": 7, 
                "other_access": 7, 
                "owner": "admin", 
                "owner_access": 7
            }, 
            "user_visible": true, 
            "uuid": {
                "uuid_lslong": 13199297585970018406, 
                "uuid_mslong": 6163359234283358722
            }
        }, 
        "instance_ip_back_refs": [
            {
                "attr": null, 
                "href": "http://172.27.113.202:8082/instance-ip/447a6191-d582-4c8e-9d58-248f65634523", 
                "to": [
                    "447a6191-d582-4c8e-9d58-248f65634523"
                ], 
                "uuid": "447a6191-d582-4c8e-9d58-248f65634523"
            }, 
            {
                "attr": null, 
                "href": "http://172.27.113.202:8082/instance-ip/94012d38-bbe7-41f7-851b-2b233e5f1ab9", 
                "to": [
                    "94012d38-bbe7-41f7-851b-2b233e5f1ab9"
                ], 
                "uuid": "94012d38-bbe7-41f7-851b-2b233e5f1ab9"
            }
        ], 
        "name": "5588a691-9695-4a02-b72d-5338b869f866", 
        "parent_href": "http://172.27.113.202:8082/project/3d4496ba-5184-4ce4-b28e-6c1885268d62", 
        "parent_type": "project", 
        "parent_uuid": "3d4496ba-5184-4ce4-b28e-6c1885268d62", 
        "routing_instance_refs": [
            {
                "attr": {
                    "direction": "both", 
                    "dst_mac": null, 
                    "mpls_label": null, 
                    "protocol": null, 
                    "service_chain_address": null, 
                    "src_mac": null, 
                    "vlan_tag": null
                }, 
                "href": "http://172.27.113.202:8082/routing-instance/1ca0b411-6955-46c9-b413-46c6d1f4bcd6", 
                "to": [
                    "default-domain", 
                    "unregard1", 
                    "ipchange1", 
                    "ipchange1"
                ], 
                "uuid": "1ca0b411-6955-46c9-b413-46c6d1f4bcd6"
            }
        ], 
        "security_group_refs": [
            {
                "attr": null, 
                "href": "http://172.27.113.202:8082/security-group/32f8679f-54b4-44d6-a165-6593e15a0bee", 
                "to": [
                    "default-domain", 
                    "unregard1", 
                    "default"
                ], 
                "uuid": "32f8679f-54b4-44d6-a165-6593e15a0bee"
            }
        ], 
        "uuid": "5588a691-9695-4a02-b72d-5338b869f866", 
        "virtual_machine_interface_allowed_address_pairs": {}, 
        "virtual_machine_interface_device_owner": "compute:nova", 
        "virtual_machine_interface_dhcp_option_list": {}, 
        "virtual_machine_interface_mac_addresses": {
            "mac_address": [
                "02:55:88:a6:91:96"
            ]
        }, 
        "virtual_machine_interface_properties": {}, 
        "virtual_machine_refs": [
            {
                "attr": null, 
                "href": "http://172.27.113.202:8082/virtual-machine/c22a52b9-b7f7-4837-923d-0da850d302ac", 
                "to": [
                    "c22a52b9-b7f7-4837-923d-0da850d302ac"
                ], 
                "uuid": "c22a52b9-b7f7-4837-923d-0da850d302ac"
            }
        ], 
        "virtual_network_refs": [
            {
                "attr": null, 
                "href": "http://172.27.113.202:8082/virtual-network/d7816f14-85e6-4efb-b211-5b61d677e7ce", 
                "to": [
                    "default-domain", 
                    "unregard1", 
                    "ipchange1"
                ], 
                "uuid": "d7816f14-85e6-4efb-b211-5b61d677e7ce"
            }
        ]
    }
}