diff --git a/src/Module/Directory/Infrastructure/ApiPlatform/State/ConvertProspectProcessor.php b/src/Module/Directory/Infrastructure/ApiPlatform/State/ConvertProspectProcessor.php
index 7c62c93..94e5067 100644
--- a/src/Module/Directory/Infrastructure/ApiPlatform/State/ConvertProspectProcessor.php
+++ b/src/Module/Directory/Infrastructure/ApiPlatform/State/ConvertProspectProcessor.php
@@ -6,7 +6,10 @@ namespace App\Module\Directory\Infrastructure\ApiPlatform\State;
 
 use ApiPlatform\Metadata\Operation;
 use ApiPlatform\State\ProcessorInterface;
+use App\Module\Directory\Domain\Entity\Address;
 use App\Module\Directory\Domain\Entity\Client;
+use App\Module\Directory\Domain\Entity\CommercialReport;
+use App\Module\Directory\Domain\Entity\Contact;
 use App\Module\Directory\Domain\Entity\Prospect;
 use App\Module\Directory\Domain\Enum\ProspectStatus;
 use App\Module\Directory\Domain\Repository\ProspectRepositoryInterface;
@@ -14,11 +17,10 @@ use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 /**
- * Converts a Prospect into a Client.
+ * Converts a Prospect into a Client and reassigns its contacts, addresses and
+ * commercial reports to the new client (preserving the commercial history).
  *
- * Loads the Prospect via the URI id, creates a Client (name = company or name,
- * copying contact details), links it back via convertedClient and flags the
- * prospect as Won. Idempotent: if already converted, returns it unchanged.
+ * Idempotent: if already converted, returns it unchanged.
  *
  * @implements ProcessorInterface<Prospect, Prospect>
  */
@@ -50,6 +52,10 @@ final readonly class ConvertProspectProcessor implements ProcessorInterface
 
         $this->entityManager->persist($client);
 
+        $this->reassignContacts($prospect, $client);
+        $this->reassignAddresses($prospect, $client);
+        $this->reassignReports($prospect, $client);
+
         $prospect->setConvertedClient($client);
         $prospect->setStatus(ProspectStatus::Won);
 
@@ -57,4 +63,28 @@ final readonly class ConvertProspectProcessor implements ProcessorInterface
 
         return $prospect;
     }
+
+    private function reassignContacts(Prospect $prospect, Client $client): void
+    {
+        foreach ($this->entityManager->getRepository(Contact::class)->findBy(['prospect' => $prospect]) as $contact) {
+            $contact->setClient($client);
+            $contact->setProspect(null);
+        }
+    }
+
+    private function reassignAddresses(Prospect $prospect, Client $client): void
+    {
+        foreach ($this->entityManager->getRepository(Address::class)->findBy(['prospect' => $prospect]) as $address) {
+            $address->setClient($client);
+            $address->setProspect(null);
+        }
+    }
+
+    private function reassignReports(Prospect $prospect, Client $client): void
+    {
+        foreach ($this->entityManager->getRepository(CommercialReport::class)->findBy(['prospect' => $prospect]) as $report) {
+            $report->setClient($client);
+            $report->setProspect(null);
+        }
+    }
 }
diff --git a/src/Module/Directory/Infrastructure/Mcp/Tool/ConvertProspectTool.php b/src/Module/Directory/Infrastructure/Mcp/Tool/ConvertProspectTool.php
index b52595b..a049837 100644
--- a/src/Module/Directory/Infrastructure/Mcp/Tool/ConvertProspectTool.php
+++ b/src/Module/Directory/Infrastructure/Mcp/Tool/ConvertProspectTool.php
@@ -4,7 +4,11 @@ declare(strict_types=1);
 
 namespace App\Module\Directory\Infrastructure\Mcp\Tool;
 
+use App\Module\Directory\Domain\Entity\Address;
 use App\Module\Directory\Domain\Entity\Client;
+use App\Module\Directory\Domain\Entity\CommercialReport;
+use App\Module\Directory\Domain\Entity\Contact;
+use App\Module\Directory\Domain\Entity\Prospect;
 use App\Module\Directory\Domain\Enum\ProspectStatus;
 use App\Module\Directory\Domain\Repository\ProspectRepositoryInterface;
 use App\Shared\Infrastructure\Mcp\Serializer;
@@ -44,6 +48,10 @@ class ConvertProspectTool
 
             $this->entityManager->persist($client);
 
+            $this->reassignContacts($prospect, $client);
+            $this->reassignAddresses($prospect, $client);
+            $this->reassignReports($prospect, $client);
+
             $prospect->setConvertedClient($client);
             $prospect->setStatus(ProspectStatus::Won);
 
@@ -52,4 +60,28 @@ class ConvertProspectTool
 
         return json_encode(Serializer::prospect($prospect));
     }
+
+    private function reassignContacts(Prospect $prospect, Client $client): void
+    {
+        foreach ($this->entityManager->getRepository(Contact::class)->findBy(['prospect' => $prospect]) as $contact) {
+            $contact->setClient($client);
+            $contact->setProspect(null);
+        }
+    }
+
+    private function reassignAddresses(Prospect $prospect, Client $client): void
+    {
+        foreach ($this->entityManager->getRepository(Address::class)->findBy(['prospect' => $prospect]) as $address) {
+            $address->setClient($client);
+            $address->setProspect(null);
+        }
+    }
+
+    private function reassignReports(Prospect $prospect, Client $client): void
+    {
+        foreach ($this->entityManager->getRepository(CommercialReport::class)->findBy(['prospect' => $prospect]) as $report) {
+            $report->setClient($client);
+            $report->setProspect(null);
+        }
+    }
 }
diff --git a/tests/Module/Directory/ConvertProspectProcessorTest.php b/tests/Module/Directory/ConvertProspectProcessorTest.php
new file mode 100644
index 0000000..c232318
--- /dev/null
+++ b/tests/Module/Directory/ConvertProspectProcessorTest.php
@@ -0,0 +1,67 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Module\Directory;
+
+use ApiPlatform\Metadata\Post;
+use App\Module\Directory\Domain\Entity\Address;
+use App\Module\Directory\Domain\Entity\CommercialReport;
+use App\Module\Directory\Domain\Entity\Contact;
+use App\Module\Directory\Domain\Entity\Prospect;
+use App\Module\Directory\Domain\Enum\ReportType;
+use App\Module\Directory\Infrastructure\ApiPlatform\State\ConvertProspectProcessor;
+use DateTimeImmutable;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+
+/**
+ * @internal
+ */
+final class ConvertProspectProcessorTest extends KernelTestCase
+{
+    public function testConvertReassignsContactsAddressesAndReports(): void
+    {
+        self::bootKernel();
+
+        /** @var EntityManagerInterface $em */
+        $em = self::getContainer()->get(EntityManagerInterface::class);
+
+        $prospect = new Prospect();
+        $prospect->setName('Atelier Test');
+        $prospect->setCompany('Atelier Test SARL');
+        $em->persist($prospect);
+
+        $contact = new Contact()->setLastName('Durand')->setProspect($prospect);
+        $address = new Address()->setCity('Niort')->setProspect($prospect);
+        $report  = new CommercialReport()
+            ->setSubject('Premier contact')
+            ->setOccurredAt(new DateTimeImmutable('2026-06-01'))
+            ->setType(ReportType::Call)
+            ->setProspect($prospect)
+        ;
+        $em->persist($contact);
+        $em->persist($address);
+        $em->persist($report);
+        $em->flush();
+
+        $processor = self::getContainer()->get(ConvertProspectProcessor::class);
+        $operation = new Post();
+        $processor->process($prospect, $operation, ['id' => $prospect->getId()]);
+
+        $em->refresh($contact);
+        $em->refresh($address);
+        $em->refresh($report);
+
+        $client = $prospect->getConvertedClient();
+        self::assertNotNull($client, 'Prospect should be converted to a client');
+
+        // Invariants (pas de counts absolus) : chaque sous-entité pointe vers le client, plus vers le prospect.
+        self::assertSame($client->getId(), $contact->getClient()?->getId());
+        self::assertNull($contact->getProspect());
+        self::assertSame($client->getId(), $address->getClient()?->getId());
+        self::assertNull($address->getProspect());
+        self::assertSame($client->getId(), $report->getClient()?->getId());
+        self::assertNull($report->getProspect());
+    }
+}