diff --git a/concept/service.go b/concept/service.go
index d955feb..43a6c5f 100644
--- a/concept/service.go
+++ b/concept/service.go
@@ -399,20 +399,39 @@ func (s *AggregateService) getConcordedConcept(ctx context.Context, UUID string,
 
 		}
 	}
-
+	var primaryOldConcept ontology.OldConcept
+	var foundPrimary bool
 	if primaryAuthority != "" {
 		canonicalConcept := bucketedConcordances[primaryAuthority][0]
-		var found bool
-		var primaryConcept ontology.OldConcept
-		found, primaryConcept, transactionID, err = s.s3.GetConceptAndTransactionID(ctx, canonicalConcept.UUID)
+		foundPrimary, primaryOldConcept, transactionID, err = s.s3.GetConceptAndTransactionID(ctx, canonicalConcept.UUID)
 		if err != nil {
 			return ontology.OldConcordedConcept{}, "", err
-		} else if !found {
+		} else if !foundPrimary {
 			err = fmt.Errorf("canonical concept %s not found in S3", canonicalConcept.UUID)
 			logger.WithField("UUID", UUID).Error(err.Error())
 			return ontology.OldConcordedConcept{}, "", err
 		}
-		oldConcepts = append(oldConcepts, primaryConcept)
+	}
+
+	// transform concepts to the new format
+	if primaryOldConcept.UUID == "" {
+		// there is no primary authority concept
+		sourceCount := len(oldConcepts)
+		if sourceCount == 0 {
+			// sanity check. concordances gathering should return 404 if there are no sources.
+			// we don't return an error in order to keep the same behavior as in v1.23 of the service.
+			logger.WithTransactionID(transactionID).WithUUID(UUID).Error("no sources found")
+			return ontology.OldConcordedConcept{}, "", nil
+		}
+		// set the primary concept to the last source concept to keep the behaviour the same as in v1.23
+		primaryOldConcept = oldConcepts[sourceCount-1]
+		oldConcepts = oldConcepts[:sourceCount-1]
+	}
+
+	primaryConcept, err := primaryOldConcept.ToSourceConcept()
+	if err != nil {
+		logger.WithError(err).WithTransactionID(transactionID).WithUUID(primaryOldConcept.UUID).Error("failed to transform primary concept to new format")
+		return ontology.OldConcordedConcept{}, "", err
 	}
 
 	var sources []ontology.SourceConcept
@@ -424,7 +443,7 @@ func (s *AggregateService) getConcordedConcept(ctx context.Context, UUID string,
 		}
 		sources = append(sources, sourceConcept)
 	}
-	concordedConcept := ontology.CreateAggregateConcept(sources)
+	concordedConcept := ontology.CreateAggregateConcept(primaryConcept, sources)
 	oldConcorded, err := concordedConcept.ToOldConcordedConcept()
 	if err != nil {
 		logger.WithError(err).WithTransactionID(transactionID).WithUUID(concordedConcept.PrefUUID).Error("failed to transform concorded concept to old format")
diff --git a/ontology/aggregate.go b/ontology/aggregate.go
index 4039713..31217e1 100644
--- a/ontology/aggregate.go
+++ b/ontology/aggregate.go
@@ -9,14 +9,21 @@ const (
 	ManagedLocationAuthority = "ManagedLocation"
 )
 
-func CreateAggregateConcept(sources []SourceConcept) ConcordedConcept {
+// CreateAggregateConcept creates ConcordedConcept by merging properties and relationships from primary and others SourceConcept
+// When merging the data from the primary SourceConcept takes precedent.
+// So if a property is present in both "primary" and one or more "other" SourceConcept,
+// the data from the primary will be in the ConcordedConcept
+// Exception to this rule are relationships that are with MergingStrategy: AggregateStrategy.
+// Those relationships will be collected from both primary and others SourceConcept into ConcordedConcept
+func CreateAggregateConcept(primary SourceConcept, others []SourceConcept) ConcordedConcept {
 	var scopeNoteOptions = map[string][]string{}
 	concordedConcept := ConcordedConcept{}
 	concordedConcept.Fields = map[string]interface{}{} // initialise Fields to be able to safely access it later
-	for _, src := range sources {
+	for _, src := range others {
 		concordedConcept = mergeCanonicalInformation(concordedConcept, src, scopeNoteOptions)
 	}
 
+	concordedConcept = mergeCanonicalInformation(concordedConcept, primary, scopeNoteOptions)
 	concordedConcept.Aliases = deduplicateAndSkipEmptyAliases(concordedConcept.Aliases)
 	concordedConcept.ScopeNote = chooseScopeNote(concordedConcept, scopeNoteOptions)
 	return concordedConcept
diff --git a/ontology/aggregate_test.go b/ontology/aggregate_test.go
index 762aa8f..1bc932a 100644
--- a/ontology/aggregate_test.go
+++ b/ontology/aggregate_test.go
@@ -25,7 +25,8 @@ func TestCreateAggregateConcept(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			sources := readSourcesFixture(t, test.Sources)
 			expected := readAggregateFixture(t, test.Aggregate)
-			actual := CreateAggregateConcept(sources)
+			primary := sources[len(sources)-1]
+			actual := CreateAggregateConcept(primary, sources[:len(sources)-1])
 			sortAliases(&expected)
 			sortAliases(&actual)
 			if !cmp.Equal(expected, actual) {
@@ -68,7 +69,8 @@ func TestCreateAggregateConcept_WithDummyConfig(t *testing.T) {
 
 	sources := readSourcesFixture(t, test.Sources)
 	expected := readAggregateFixture(t, test.Aggregate)
-	actual := CreateAggregateConcept(sources)
+	primary := sources[len(sources)-1]
+	actual := CreateAggregateConcept(primary, sources[:len(sources)-1])
 	sortAliases(&expected)
 	sortAliases(&actual)
 	if !cmp.Equal(expected, actual) {