Label nodes in your node pools:
NodePool1: nodepool=nodepool1
NodePool2: nodepool=nodepool2

Update the StatefulSet to add affinity to force Pods to run only on nodepool2.

spec:
  template:
    spec:
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
              - matchExpressions:
                  - key: nodepool
                    operator: In
                    values:
                      - nodepool2

If you have a StatefulSet deployed across two node pools (nodepool1 and nodepool2) in your aks cluster

and you want to scale it down to 3 replicas, you can taint nodepool1’s nodes to prevent any pods from being scheduled on it.

kubectl taint nodes <node-name> nodepool=nodepool1:NoSchedule

Edit your Stateful Set yaml to include a nodeAffinity rule to ensure all pods are rescheduled to nodepool2.

affinity:
  nodeAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
      nodeSelectorTerms:
      - matchExpressions:
        - key: agentpool
          operator: In
          values:
          - nodepool2

Cordon the drain the nodes to prevent new pods from being scheduled

kubectl cordon <node-name>

Once all pods are running on nodepool2, scale down the StatefulSet to 3 replicas

kubectl scale statefulset <statefulset-name> --replicas=3

Verify the pods now, it should show 3 replicas and all on nodepool2

kubectl get pods -o wide

StatefulSets do not delete PersistentVolumeClaims (PVCs) for scaled-down replicas automatically so you gotta delete those yourself and then check

kubectl get pvc

looks good

Relevant MS docs-

• Resize node pools in Azure Kubernetes Service (AKS)
• Manually scale the node count in an Azure Kubernetes Service (AKS) cluster