terraform-provider-azurerm: Modifying node pool for deprecated k8s version is not possible

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Terraform (and AzureRM Provider) Version

Terraform v0.12.26

• provider.azuread v0.11.0
• provider.azurerm v2.18.0
• provider.helm v1.2.4
• provider.kubernetes v1.12.0
• provider.random v2.3.0

Affected Resource(s)

azurerm_kubernetes_cluster

Terraform Configuration Files

No special configuration is necessary but you must have an AKS using a version of k8s that is no longer supported.

Debug Output

I have debug output but haven’t removed any sensitive data from it. If it’s necessary to share, I will do so.

Expected Behavior

Terraform modification of a node pool using an unsupported k8s version should be successful.

Actual Behavior

An error is displayed:

The Kubernetes/Orchestrator Version “1.16.8” is not available for Node Pool “default”.

Please confirm that this version is supported by the Kubernetes Cluster “aks-staging” (Resource Group “rg-staging”) - which may need to be upgraded first.

The Kubernetes Cluster is running version “1.16.8”.

The supported Orchestrator Versions for this Node Pool/supported by this Kubernetes Cluster are:

• 1.15.12
• 1.15.11

Steps to Reproduce

1. Hopefully you already have an AKS deployed for an unsupported version. I was using 1.16.8.
2. Modify your terraform script to make a change to the node pool (change the node count).
3. Apply

Important Factoids

This error appears to have been introduced with the June AKS updates.

One thing in particular to call out is updating Node Pools - where a Kubernetes Cluster/Control Plane must be updated before the Node Pools can be updated

I hope this isn’t meant to imply that you must be on a supported k8s version in order to make modifications through terraform. I can still make modifications to my unsupported cluster version through the Portal UI. I would think this is a pretty common scenario where a tweak may need to be done and upgrading isn’t possible due to the impact a k8s upgrade has on the cluster and deployed services.