diff --git a/src/Controller/ComposantPieceSlotController.php b/src/Controller/ComposantPieceSlotController.php
index 522a6f6..6af2f9b 100644
--- a/src/Controller/ComposantPieceSlotController.php
+++ b/src/Controller/ComposantPieceSlotController.php
@@ -43,6 +43,18 @@ class ComposantPieceSlotController extends AbstractController
                 $slot->setSelectedPiece(null);
             } else {
                 $piece = $this->entityManager->find(Piece::class, $payload['selectedPieceId']);
+                if (!$piece) {
+                    return $this->json(['success' => false, 'error' => 'Pièce introuvable.'], 404);
+                }
+
+                $slotTypePiece = $slot->getTypePiece();
+                if ($slotTypePiece && $piece->getTypePiece()?->getId() !== $slotTypePiece->getId()) {
+                    return $this->json([
+                        'success' => false,
+                        'error'   => sprintf('La pièce doit être de type « %s ».', $slotTypePiece->getName()),
+                    ], 422);
+                }
+
                 $slot->setSelectedPiece($piece);
             }
         }
diff --git a/src/Controller/CustomFieldValueController.php b/src/Controller/CustomFieldValueController.php
index 162ee4f..54ee926 100644
--- a/src/Controller/CustomFieldValueController.php
+++ b/src/Controller/CustomFieldValueController.php
@@ -196,19 +196,16 @@ class CustomFieldValueController extends AbstractController
             return $this->json(['success' => false, 'error' => 'customFieldId or customFieldName is required.'], 400);
         }
 
-        $customField = new CustomField();
-        $customField->setName($customFieldName);
-        $customField->setType((string) ($payload['customFieldType'] ?? 'text'));
-        $customField->setRequired((bool) ($payload['customFieldRequired'] ?? false));
-
-        $options = $payload['customFieldOptions'] ?? null;
-        if (is_array($options)) {
-            $customField->setOptions($options);
+        // Try to find an existing custom field by name instead of creating an orphan
+        $existing = $this->customFieldRepository->findOneBy(['name' => $customFieldName]);
+        if ($existing instanceof CustomField) {
+            return $existing;
         }
 
-        $this->entityManager->persist($customField);
-
-        return $customField;
+        return $this->json([
+            'success' => false,
+            'error'   => sprintf('Custom field "%s" not found. Create it explicitly first.', $customFieldName),
+        ], 404);
     }
 
     private function resolveTarget(array $payload): array|JsonResponse
diff --git a/src/Controller/MachineStructureController.php b/src/Controller/MachineStructureController.php
index 20c8d1c..1986d32 100644
--- a/src/Controller/MachineStructureController.php
+++ b/src/Controller/MachineStructureController.php
@@ -173,6 +173,8 @@ class MachineStructureController extends AbstractController
 
     private function cloneCustomFields(Machine $source, Machine $target): void
     {
+        $cfMap = [];
+
         foreach ($source->getCustomFields() as $cf) {
             $newCf = new CustomField();
             $newCf->setName($cf->getName());
@@ -183,12 +185,20 @@ class MachineStructureController extends AbstractController
             $newCf->setOrderIndex($cf->getOrderIndex());
             $newCf->setMachine($target);
             $this->entityManager->persist($newCf);
+
+            $cfMap[$cf->getId()] = $newCf;
         }
 
         foreach ($source->getCustomFieldValues() as $cfv) {
+            $originalCf = $cfv->getCustomField();
+            $newCf      = $cfMap[$originalCf->getId()] ?? null;
+            if (!$newCf) {
+                continue;
+            }
+
             $newValue = new CustomFieldValue();
             $newValue->setMachine($target);
-            $newValue->setCustomField($cfv->getCustomField());
+            $newValue->setCustomField($newCf);
             $newValue->setValue($cfv->getValue());
             $this->entityManager->persist($newValue);
         }
diff --git a/src/Service/ModelTypeCategoryConversionService.php b/src/Service/ModelTypeCategoryConversionService.php
index e91bf27..8942b5d 100644
--- a/src/Service/ModelTypeCategoryConversionService.php
+++ b/src/Service/ModelTypeCategoryConversionService.php
@@ -235,6 +235,45 @@ final class ModelTypeCategoryConversionService
             }
         }
 
+        // Check slot tables for actual data (post-normalization architecture)
+        $filledPieceSlots = (int) $this->connection->fetchOne(
+            'SELECT COUNT(*) FROM composant_piece_slots cps
+             JOIN composants c ON cps.composantid = c.id
+             WHERE c.typecomposantid = :id AND cps.selectedpieceid IS NOT NULL',
+            ['id' => $modelTypeId],
+        );
+
+        $filledSubSlots = (int) $this->connection->fetchOne(
+            'SELECT COUNT(*) FROM composant_subcomponent_slots css
+             JOIN composants c ON css.composantid = c.id
+             WHERE c.typecomposantid = :id AND css.selectedcomposantid IS NOT NULL',
+            ['id' => $modelTypeId],
+        );
+
+        $filledProductSlots = (int) $this->connection->fetchOne(
+            'SELECT COUNT(*) FROM composant_product_slots cps
+             JOIN composants c ON cps.composantid = c.id
+             WHERE c.typecomposantid = :id AND cps.selectedproductid IS NOT NULL',
+            ['id' => $modelTypeId],
+        );
+
+        if ($filledPieceSlots > 0 || $filledSubSlots > 0 || $filledProductSlots > 0) {
+            $parts = [];
+            if ($filledPieceSlots > 0) {
+                $parts[] = sprintf('%d slot(s) pièce', $filledPieceSlots);
+            }
+            if ($filledSubSlots > 0) {
+                $parts[] = sprintf('%d slot(s) sous-composant', $filledSubSlots);
+            }
+            if ($filledProductSlots > 0) {
+                $parts[] = sprintf('%d slot(s) produit', $filledProductSlots);
+            }
+            $blockers[] = sprintf(
+                'Des composants ont des données dans leurs slots : %s.',
+                implode(', ', $parts),
+            );
+        }
+
         // Check name collision with existing pieces
         $collisions = $this->connection->fetchFirstColumn(
             'SELECT c.name FROM composants c
@@ -383,12 +422,22 @@ final class ModelTypeCategoryConversionService
             ['id' => $modelTypeId],
         );
 
-        // 6. Delete original composants
+        // 6. Delete original composants (cascades to slot tables)
         $this->connection->executeStatement(
             'DELETE FROM composants WHERE typecomposantid = :id',
             ['id' => $modelTypeId],
         );
 
+        // 6b. Clean up skeleton requirements that only apply to COMPONENT category
+        $this->connection->executeStatement(
+            'DELETE FROM skeleton_piece_requirements WHERE modeltypeid = :id',
+            ['id' => $modelTypeId],
+        );
+        $this->connection->executeStatement(
+            'DELETE FROM skeleton_subcomponent_requirements WHERE modeltypeid = :id',
+            ['id' => $modelTypeId],
+        );
+
         // 7. Update ModelType
         $this->connection->executeStatement(
             'UPDATE model_types