Update src/aks-preview/azext_aks_preview/custom.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: m-chau

src/aks-preview/azext_aks_preview/custom.py

@@ -1724,16 +1724,16 @@ def aks_upgrade(cmd,
 
     if tier is not None:
         instance.sku.tier = tier
-        
+
     if k8s_support_plan is not None:
         instance.support_plan = k8s_support_plan
-        if (
-            instance.support_plan == KubernetesSupportPlan.AKS_LONG_TERM_SUPPORT
-            and instance.sku.tier is not None
-            and instance.sku.tier.lower() != CONST_MANAGED_CLUSTER_SKU_TIER_PREMIUM.lower()
-        ):
-            raise CLIError("AKS Long Term Support is only available for Premium tier clusters.")
 
+    if (
+        instance.support_plan == KubernetesSupportPlan.AKS_LONG_TERM_SUPPORT
+        and instance.sku.tier is not None
+        and instance.sku.tier.lower() != CONST_MANAGED_CLUSTER_SKU_TIER_PREMIUM.lower()
+    ):
+        raise CLIError("AKS Long Term Support is only available for Premium tier clusters.")
     if instance.kubernetes_version == kubernetes_version:
         if instance.provisioning_state == "Succeeded":
             logger.warning("The cluster is already on version %s and is not in a failed state. No operations "