Merge pull request #278284 from dramasamy/upgradeos

Added a note about no OS image change

Author: prmerger-automator[bot]

articles/operator-nexus/howto-kubernetes-cluster-upgrade.md

@@ -90,6 +90,9 @@ During the cluster upgrade process, Operator Nexus performs the following operat
 * After the old worker node has been drained, it's removed, and a new worker node with the new Kubernetes version is added to the cluster (or as many nodes as configured in [max surge](#customize-node-surge-upgrade))
 * This process repeats until all worker nodes in the cluster have been upgraded.
 
+> [!NOTE]
+> The cluster upgrade may not create new nodes and replace the old ones if the operating system (OS) image version and Kubernetes version remain the same between version bundles. This is expected behavior, as the upgrade may only include updates to Addon versions rather than new OS or K8s versions. Since there is no rolling upgrade involved, there is no cordon and drain on the nodes, so Pod disruptions will not occur.
+
 > [!IMPORTANT]
 > Ensure that any `PodDisruptionBudgets` ([PDB](https://kubernetes.io/docs/concepts/workloads/pods/disruptions/#pod-disruption-budgets)) allow for at least *one* pod replica to be moved at a time otherwise the drain/evict operation will fail.
 > If the drain operation fails, the upgrade operation will fail as well, to ensure that the applications are not disrupted. Please correct what caused the operation to stop (i.e. incorrect PDBs, lack of quota, etc.) and re-try the operation.