diff --git a/pkg/cluster/cluster.go b/pkg/cluster/cluster.go
index b9759ca86..03fca8908 100644
--- a/pkg/cluster/cluster.go
+++ b/pkg/cluster/cluster.go
@@ -576,6 +576,14 @@ func (c *Cluster) Update(oldSpec, newSpec *acidv1.Postgresql) error {
 				c.logger.Errorf("could not sync statefulsets: %v", err)
 				updateFailed = true
 			}
+		} else {
+			// patroni configuration is not always reflected on the StatefulSet as they have
+			// different lifecycle (due to SPILO's implementation - see: issues#501) but they
+			// still need to be updated via the Patroni REST API.
+			if err := c.applyPatroniConfig(); err != nil {
+				c.logger.Errorf("could not apply patroni config: %v", err)
+				updateFailed = true
+			}
 		}
 	}()
 
diff --git a/pkg/cluster/k8sres.go b/pkg/cluster/k8sres.go
index a16e810ed..6b5c05d27 100644
--- a/pkg/cluster/k8sres.go
+++ b/pkg/cluster/k8sres.go
@@ -149,7 +149,7 @@ func fillResourceList(spec acidv1.ResourceDescription, defaults acidv1.ResourceD
 	return requests, nil
 }
 
-func generateSpiloJSONConfiguration(pg *acidv1.PostgresqlParam, patroni *acidv1.Patroni, pamRoleName string, logger *logrus.Entry) (string, error) {
+func generateSpiloJSONConfiguration(pg *acidv1.PostgresqlParam, patroni *acidv1.Patroni, pamRoleName string, logger *logrus.Entry, isNewCluster bool, initialPatroniConfig string) (string, error) {
 	config := spiloConfiguration{}
 
 	config.Bootstrap = pgBootstrap{}
@@ -219,8 +219,27 @@ PatroniInitDBParams:
 	if patroni.TTL != 0 {
 		config.Bootstrap.DCS.TTL = patroni.TTL
 	}
-	if patroni.Slots != nil {
-		config.Bootstrap.DCS.Slots = patroni.Slots
+
+	// The SPILO image is taking in account the 'Slots' settings **only** when bootstraping a new cluster.
+	// Therefore, updating this value in the SPILO_CONFIGURATION of the statefulset is pointless, but still
+	// required at cluster creation.
+	if isNewCluster {
+		if patroni.Slots != nil {
+			config.Bootstrap.DCS.Slots = patroni.Slots
+		}
+	} else {
+		oldestKnownPatroniCfg := acidv1.Patroni{}
+		if initialPatroniConfig == "" {
+			// when upgrading postgres-operator to this new version, existing StatefulSet won't have this annotation, so we're using the current specs
+			oldestKnownPatroniCfg = *patroni
+		} else {
+			if err := json.Unmarshal([]byte(initialPatroniConfig), &oldestKnownPatroniCfg); err != nil {
+				logger.Errorf("cannot decode initial patroni config from annotations: %s (%v)", err, initialPatroniConfig)
+				return "", err
+			}
+		}
+
+		config.Bootstrap.DCS.Slots = oldestKnownPatroniCfg.Slots
 	}
 
 	config.PgLocalConfiguration = make(map[string]interface{})
@@ -784,7 +803,12 @@ func (c *Cluster) generateStatefulSet(spec *acidv1.PostgresSpec) (*v1beta1.State
 			func(i, j int) bool { return customPodEnvVarsList[i].Name < customPodEnvVarsList[j].Name })
 	}
 
-	spiloConfiguration, err := generateSpiloJSONConfiguration(&spec.PostgresqlParam, &spec.Patroni, c.OpConfig.PamRoleName, c.logger)
+	// c.Statefulset is not set when called from .createStatefulSet()
+	initialPatroniConfig := ""
+	if c.Statefulset != nil {
+		initialPatroniConfig = c.Statefulset.ObjectMeta.Annotations[initialPatroniConfigAnnotationKey]
+	}
+	spiloConfiguration, err := generateSpiloJSONConfiguration(&spec.PostgresqlParam, &spec.Patroni, c.OpConfig.PamRoleName, c.logger, c.isNewCluster(), initialPatroniConfig)
 	if err != nil {
 		return nil, fmt.Errorf("could not generate Spilo JSON configuration: %v", err)
 	}
@@ -875,6 +899,20 @@ func (c *Cluster) generateStatefulSet(spec *acidv1.PostgresSpec) (*v1beta1.State
 
 	numberOfInstances := c.getNumberOfInstances(spec)
 
+	ann := map[string]string{rollingUpdateStatefulsetAnnotationKey: "false"}
+	// Save the initial patroni config for later usage.
+	// Note: if the annotation is empty (during the upgrade path for example), set it to the current specs.
+	// see: issues#501
+	if c.isNewCluster() || initialPatroniConfig == "" {
+		x, err := json.Marshal(spec.Patroni)
+		if err != nil {
+			return nil, fmt.Errorf("could not JSON encode initial patroni config: %v", err)
+		}
+		ann[initialPatroniConfigAnnotationKey] = string(x)
+	} else {
+		ann[initialPatroniConfigAnnotationKey] = initialPatroniConfig
+	}
+
 	// the operator has domain-specific logic on how to do rolling updates of PG clusters
 	// so we do not use default rolling updates implemented by stateful sets
 	// that leaves the legacy "OnDelete" update strategy as the only option
@@ -894,7 +932,7 @@ func (c *Cluster) generateStatefulSet(spec *acidv1.PostgresSpec) (*v1beta1.State
 			Name:        c.statefulSetName(),
 			Namespace:   c.Namespace,
 			Labels:      c.labelsSet(true),
-			Annotations: map[string]string{rollingUpdateStatefulsetAnnotationKey: "false"},
+			Annotations: ann,
 		},
 		Spec: v1beta1.StatefulSetSpec{
 			Replicas:             &numberOfInstances,
@@ -1242,11 +1280,11 @@ func (c *Cluster) generateCloneEnvironment(description *acidv1.CloneDescription)
 			c.logger.Info(msg, description.S3WalPath)
 
 			envs := []v1.EnvVar{
-				v1.EnvVar{
+				{
 					Name:  "CLONE_WAL_S3_BUCKET",
 					Value: c.OpConfig.WALES3Bucket,
 				},
-				v1.EnvVar{
+				{
 					Name:  "CLONE_WAL_BUCKET_SCOPE_SUFFIX",
 					Value: getBucketScopeSuffix(description.UID),
 				},
diff --git a/pkg/cluster/resources.go b/pkg/cluster/resources.go
index c89edac63..c3f25e51b 100644
--- a/pkg/cluster/resources.go
+++ b/pkg/cluster/resources.go
@@ -20,6 +20,7 @@ import (
 
 const (
 	rollingUpdateStatefulsetAnnotationKey = "zalando-postgres-operator-rolling-update-required"
+	initialPatroniConfigAnnotationKey     = "v1.acid.zalan.do/initial-patroni-config"
 )
 
 func (c *Cluster) listResources() error {
diff --git a/pkg/cluster/sync.go b/pkg/cluster/sync.go
index f5ae30b81..2d47f57c6 100644
--- a/pkg/cluster/sync.go
+++ b/pkg/cluster/sync.go
@@ -314,6 +314,10 @@ func (c *Cluster) syncStatefulSet() error {
 		return fmt.Errorf("could not set cluster-wide PostgreSQL configuration options: %v", err)
 	}
 
+	if err := c.applyPatroniConfig(); err != nil {
+		return fmt.Errorf("could not apply patroni config: %v", err)
+	}
+
 	// if we get here we also need to re-create the pods (either leftovers from the old
 	// statefulset or those that got their configuration from the outdated statefulset)
 	if podsRollingUpdateRequired {
@@ -372,6 +376,35 @@ func (c *Cluster) checkAndSetGlobalPostgreSQLConfiguration() error {
 		len(pods))
 }
 
+// TODO: merge applyPatroniConfig() and checkAndSetGlobalPostgreSQLConfiguration()?
+func (c *Cluster) applyPatroniConfig() error {
+	var (
+		err  error
+		pods []v1.Pod
+	)
+
+	if pods, err = c.listPods(); err != nil {
+		return err
+	}
+	if len(pods) == 0 {
+		return fmt.Errorf("could not call Patroni API: cluster has no pods")
+	}
+	// try all pods until the first one that is successful, as it doesn't matter which pod
+	// carries the request to change configuration through
+	for _, pod := range pods {
+		podName := util.NameFromMeta(pod.ObjectMeta)
+		c.logger.Debugf("calling Patroni API on a pod %s to set the following patroni config: %v",
+			podName, c.Postgresql.Spec.Patroni)
+
+		if err = c.patroni.ApplyConfig(&pod, c.Postgresql.Spec.Patroni); err == nil {
+			return nil
+		}
+		c.logger.Warningf("could not patch patroni config with on pod %s: %v", podName, err)
+	}
+	return fmt.Errorf("could not reach Patroni API to patch patroni config: failed on every pod (%d total)",
+		len(pods))
+}
+
 func (c *Cluster) syncSecrets() error {
 	var (
 		err    error
diff --git a/pkg/util/patroni/patroni.go b/pkg/util/patroni/patroni.go
index 23260f277..3e9e94d83 100644
--- a/pkg/util/patroni/patroni.go
+++ b/pkg/util/patroni/patroni.go
@@ -10,6 +10,8 @@ import (
 
 	"github.com/sirupsen/logrus"
 	"k8s.io/api/core/v1"
+
+	acidv1 "github.com/zalando/postgres-operator/pkg/apis/acid.zalan.do/v1"
 )
 
 const (
@@ -23,6 +25,7 @@ const (
 type Interface interface {
 	Switchover(master *v1.Pod, candidate string) error
 	SetPostgresParameters(server *v1.Pod, options map[string]string) error
+	ApplyConfig(server *v1.Pod, config acidv1.Patroni) error
 }
 
 // Patroni API client
@@ -102,3 +105,12 @@ func (p *Patroni) SetPostgresParameters(server *v1.Pod, parameters map[string]st
 	}
 	return p.httpPostOrPatch(http.MethodPatch, apiURL(server)+configPath, buf)
 }
+
+func (p *Patroni) ApplyConfig(server *v1.Pod, config acidv1.Patroni) error {
+	buf := &bytes.Buffer{}
+	err := json.NewEncoder(buf).Encode(config)
+	if err != nil {
+		return fmt.Errorf("could not encode json: %v", err)
+	}
+	return p.httpPostOrPatch(http.MethodPatch, apiURL(server)+configPath, buf)
+}