Deployment Manager cannot update instance templates - NO_METHOD_TO_UPDATE_FIELD - google-cloud-platform

Question

I have a deployment comprising a managed instance group and two instance templates (A and B). The deployment was initially created with the instance group referencing instance template A.
I tried updating the sourceImage in instance template B using deployment manager (gcloud beta deployment-manager deployments update my-deployment --template ...), but got the following error:
ERROR: (gcloud.beta.deployment-manager.deployments.update) Error in
Operation [operation-1538798895713-57787898f4ae9-8b478716-0bb72a09]:
errors:
- code: NO_METHOD_TO_UPDATE_FIELD
message: No method found to update field 'properties' on
resource 'fwp-app-preprod-instance-template-a' of type
'compute.v1.instanceTemplate'. The resource may need to be
recreated with the new field.
I should make it clear that the only change I made from the original deployment is the instance template's sourceImage.
Is it possible to perform an update of an instance template via deployment manager so that it references an updated sourceImage?
The error states clearly that the resource (instance template) may need to be recreated, and I'm happy for deployment manager to do that. But I have no idea how to instruct/force deployment manager to take that action.
I don't doubt it can be done outside of deployment manager, but I want to avoid configuration drift.
My app.jinja.schema:
imports:
- path: instance-group.jinja
- path: instance-template.jinja
My app.jinja:
resources:
- name: instance-template-a
type: instance-template.jinja
properties:
name: {{ env["deployment"] }}-instance-template-a
machineType: g1-small
sourceImage: "projects/my-project/global/images/my-image"
diskSizeGb: '30'
- name: instance-template-b
type: instance-template.jinja
properties:
name: {{ env["deployment"] }}-instance-template-b
machineType: g1-small
sourceImage: "projects/my-project/global/images/my-image"
diskSizeGb: '30'
- name: fwp-instance-group
type: instance-group.jinja
My instance-group.jinja:
resources:
- name: 'instance-group-{{ env["deployment"] }}'
type: compute.v1.regionInstanceGroupManager
properties:
baseInstanceName: ig-instance-{{ env["deployment"] }}
instanceTemplate: '$(ref.{{ env["deployment"] }}-instance-template-a.selfLink)'
targetSize: 1
region: australia-southeast1
- name: 'autoscaler-{{ env["deployment"] }}'
type: compute.v1.regionAutoscalers
properties:
autoscalingPolicy:
coolDownPeriodSec: 60
loadBalancingUtilization:
utilizationTarget: 0.9
maxNumReplicas: 10
minNumReplicas: 2
target: $(ref.instance-group-{{ env["deployment"] }}.selfLink)
region: australia-southeast1
And my instance-template.jinja
resources:
- name: {{ properties["name"] }}
type: compute.v1.instanceTemplate
properties:
name: {{ properties["name"] }}
description: ''
properties:
machineType: {{ properties["machineType"] }}
tags:
items:
- no-ip
- web-server
- http-server
- https-server
disks:
- type: 'PERSISTENT'
boot: true
mode: 'READ_WRITE'
autoDelete: true
deviceName: instance-device
initializeParams:
sourceImage: {{ properties["sourceImage"] }}
diskType: 'pd-standard'
diskSizeGb: {{ properties["diskSizeGb"] }}
canIpForward: false
networkInterfaces:
- network: projects/my-project/global/networks/vpc-fwp-nonprod
subnetwork: projects/my-project/regions/australia-southeast1/subnetworks/subnet-private-fwp-nonprod
aliasIpRanges: []
labels: { environment: {{ env["deployment"] }}, tenancy: "fwp-nonprod" }
scheduling:
preemptible: false
onHostMaintenance: MIGRATE
automaticRestart: true
nodeAffinities: []
serviceAccounts:
- email: some-service-account#developer.gserviceaccount.com
scopes:
- https://www.googleapis.com/auth/cloud-platform

Answer 1

To recap the comments:
The DM config includes an instance template for the managed instance group. The change of source image is attempting to change the image used in the template.
Unfortunately, instance templates are immutable once created
"So it is not possible to update an existing instance template or change an instance template after it has been created."
This explains the error message returned. The proper way to change the image you want to use for a Managed Instance Group is to create a new template and perform a rolling update on the group and using the new instance template.