diff --git a/hooks/rabbitmq_server_relations.py b/hooks/rabbitmq_server_relations.py
index fd75a0ab..a8a089fe 100755
--- a/hooks/rabbitmq_server_relations.py
+++ b/hooks/rabbitmq_server_relations.py
@@ -819,6 +819,12 @@ def upgrade_charm():
     if missing_packages:
         apt_install(missing_packages, fatal=True)
 
+    if is_leader() and not leader_get(rabbit.CLUSTER_MODE_KEY):
+        log("Setting {} to {} on upgrade-charm.".format(
+            rabbit.CLUSTER_MODE_KEY,
+            config(rabbit.CLUSTER_MODE_KEY)), level='INFO')
+        leader_set({rabbit.CLUSTER_MODE_KEY: config(rabbit.CLUSTER_MODE_KEY)})
+
 
 MAN_PLUGIN = 'rabbitmq_management'
 PROM_PLUGIN = 'rabbitmq_prometheus'
diff --git a/unit_tests/test_rabbitmq_server_relations.py b/unit_tests/test_rabbitmq_server_relations.py
index aa3f59e0..094adcef 100644
--- a/unit_tests/test_rabbitmq_server_relations.py
+++ b/unit_tests/test_rabbitmq_server_relations.py
@@ -479,3 +479,48 @@ class RelationUtil(CharmTestCase):
             'rabbitmq-server')
         wait_app.assert_called_once_with()
         self.assertFalse(update_clients.called)
+
+    @patch.object(rabbitmq_server_relations, 'leader_set')
+    @patch.object(rabbitmq_server_relations, 'leader_get')
+    @patch.object(rabbitmq_server_relations, 'apt_install')
+    @patch.object(rabbitmq_server_relations, 'filter_installed_packages')
+    @patch.object(rabbitmq_server_relations, 'apt_update')
+    @patch.object(rabbitmq_server_relations, 'update_clients')
+    @patch('rabbit_utils.clustered_with_leader')
+    @patch('rabbit_utils.update_peer_cluster_status')
+    @patch('rabbit_utils.migrate_passwords_to_peer_relation')
+    @patch.object(rabbitmq_server_relations, 'is_elected_leader')
+    @patch('os.listdir')
+    @patch('charmhelpers.contrib.hardening.harden.config')
+    @patch.object(rabbitmq_server_relations, 'config')
+    def test_upgrade_charm(self, config, harden_config, listdir,
+                           is_elected_releader,
+                           migrate_passwords_to_peer_relation,
+                           update_peer_cluster_status,
+                           clustered_with_leader,
+                           update_clients,
+                           apt_update,
+                           filter_installed_packages,
+                           apt_install,
+                           leader_get,
+                           leader_set):
+        config.side_effect = self.test_config
+        harden_config.side_effect = self.test_config
+        is_elected_releader.return_value = True
+        clustered_with_leader.return_value = True
+        filter_installed_packages.side_effect = lambda x: x
+
+        leader_get.return_value = None
+
+        rabbitmq_server_relations.upgrade_charm()
+
+        migrate_passwords_to_peer_relation.assert_called()
+        update_peer_cluster_status.assert_called()
+        apt_update.assert_called_with(fatal=True)
+        apt_install.assert_called_with(['python3-amqplib',
+                                        'python3-croniter'],
+                                       fatal=True)
+        leader_set.assert_called_with(
+            {rabbit_utils.CLUSTER_MODE_KEY:
+             self.test_config.get(rabbit_utils.CLUSTER_MODE_KEY)}
+        )