Merge pull request #248208 from wedaly/widaly/cilium-upgrade-docs-draft

Add docs to upgrade existing cluster to Cilium

Author: prmerger-automator[bot]

articles/aks/azure-cni-powered-by-cilium.md

@@ -110,6 +110,29 @@ az aks create -n <clusterName> -g <resourceGroupName> -l <location> \
   --network-dataplane cilium
 ```
 
+## Upgrade an existing cluster to Azure CNI Powered by Cilium
+
+> [!NOTE]
+> You can update an existing cluster to Azure CNI Powered by Cilium if the cluster meets the following criteria:
+>
+> - The cluster uses either [Azure CNI Overlay](./azure-cni-overlay.md) or [Azure CNI with dynamic IP allocation](./configure-azure-cni-dynamic-ip-allocation.md). This does **not** include [Azure CNI](./configure-azure-cni.md).
+> - The cluster does not have Azure NPM or Calico enabled.
+> - The cluster does not have any Windows node pools.
+
+The upgrade process triggers each node pool to be re-imaged simultaneously. Upgrading each node pool separately isn't supported. Any disruptions to cluster networking are similar to a node image upgrade or [Kubernetes version upgrade](./upgrade-cluster.md) where each node in a node pool is re-imaged.
+
+Cilium will begin enforcing network policies only after all nodes have been re-imaged.
+
+To perform the upgrade, you will need Azure CLI version 2.52.0 or later. Run `az --version` to see the currently installed version. If you need to install or upgrade, see [Install Azure CLI](/cli/azure/install-azure-cli).
+
+Use the following command to upgrade an existing cluster to Azure CNI Powered by Cilium. Replace the values for `<clusterName>` and `<resourceGroupName>`:
+
+```azurecli-interactive
+az aks update -n <clusterName> -g <resourceGroupName> \
+  --network-dataplane cilium
+```
+
+
 ## Frequently asked questions
 
 - **Can I customize Cilium configuration?**