Mention downtime in upgrade articles

Author: jonels-msft

articles/postgresql/hyperscale/concepts-upgrade.md

@@ -6,7 +6,7 @@ author: jonels-msft
 ms.service: postgresql
 ms.subservice: hyperscale-citus
 ms.topic: conceptual
-ms.date: 08/02/2022
+ms.date: 08/29/2022
 ---
 
 # Hyperscale (Citus) server group upgrades
@@ -20,9 +20,15 @@ higher.
 
 ## Upgrade precautions
 
-Upgrading a major version of Citus can introduce changes in behavior.
-It's best to familiarize yourself with new product features and changes
-to avoid surprises.
+Upgrades require some downtime in the database cluster. The exact time depends
+on the source and destinationn versions of the upgrade. To prepare for the
+production cluster upgrade, we recommend [testing the
+upgrade](howto-upgrade.md#test-the-upgrade-first), and measure downtime during
+the test.
+
+Also, upgrading a major version of Citus can introduce changes in behavior.
+It's best to familiarize yourself with new product features and changes to
+avoid surprises.
 
 Noteworthy Citus 11 changes:
 

articles/postgresql/hyperscale/howto-upgrade.md

@@ -6,7 +6,7 @@ author: jonels-msft
 ms.service: postgresql
 ms.subservice: hyperscale-citus
 ms.topic: how-to
-ms.date: 08/02/2022
+ms.date: 08/29/2022
 ---
 
 # Upgrade Hyperscale (Citus) server group
@@ -20,7 +20,8 @@ on all server group nodes.
 
 Upgrading PostgreSQL causes more changes than you might imagine, because
 Hyperscale (Citus) will also upgrade the [database
-extensions](reference-extensions.md), including the Citus extension.
+extensions](reference-extensions.md), including the Citus extension. Upgrades
+also require downtime in the database cluster.
 
 We strongly recommend you to test your application with the new PostgreSQL and
 Citus version before you upgrade your production environment.  Also, please see