diff --git a/README.md b/README.md
index 2ef2a3d..f095ae5 100644
--- a/README.md
+++ b/README.md
@@ -34,6 +34,11 @@ administrator. That account can sign in at `/manager/sign-in`, create
 businesses, edit their primary domains, and share the one-time owner setup
 link generated during provisioning.
 
+For an existing installation created before this platform/tenant split, the
+recorded `initialAdminUserId` account is allowed to sign in once and is
+promoted to the platform administrator. No other business owner can use that
+upgrade path.
+
 Business provisioning creates the tenant domain, settings, booking rules,
 opening hours, system roles, owner staff profile, and seven-day owner
 invitation in one transaction. The owner chooses a password through that
diff --git a/app/Http/Controllers/PlatformAuthController.php b/app/Http/Controllers/PlatformAuthController.php
index 2a8ad89..205a6fa 100644
--- a/app/Http/Controllers/PlatformAuthController.php
+++ b/app/Http/Controllers/PlatformAuthController.php
@@ -4,6 +4,7 @@ namespace App\Http\Controllers;
 
 use App\Enums\InstallationStatus;
 use App\Models\ApplicationInstallation;
+use App\Models\User;
 use Illuminate\Http\RedirectResponse;
 use Illuminate\Http\Request;
 use Illuminate\Support\Facades\Auth;
@@ -31,12 +32,34 @@ class PlatformAuthController extends Controller
             'password' => ['required', 'string'],
         ]);
 
-        if (! Auth::attempt($credentials, $request->boolean('remember')) || ! $request->user()?->isSuperAdmin) {
+        if (! Auth::attempt($credentials, $request->boolean('remember'))) {
             Auth::logout();
 
             return back()->withErrors(['email' => 'Those platform administrator details do not match our records.'])->onlyInput('email');
         }
 
+        $user = $request->user();
+        if (! $user?->isSuperAdmin) {
+            $installation = ApplicationInstallation::query()
+                ->where('status', InstallationStatus::Installed)
+                ->first();
+
+            $canUpgradeLegacyAdmin = ! User::query()->where('isSuperAdmin', true)->exists()
+                && $installation?->initialAdminUserId === $user?->id;
+
+            if (! $canUpgradeLegacyAdmin) {
+                Auth::logout();
+
+                return back()->withErrors(['email' => 'Those platform administrator details do not match our records.'])->onlyInput('email');
+            }
+
+            // Existing installations created before platform administration
+            // was separated may have only an initial business owner. Promote
+            // that exact recorded account once so the migration is usable.
+            $user->forceFill(['isSuperAdmin' => true])->save();
+            Auth::setUser($user->fresh());
+        }
+
         $request->session()->regenerate();
 
         return redirect()->intended(route('manager.dashboard'));
diff --git a/tests/Feature/InstallationTest.php b/tests/Feature/InstallationTest.php
index 6d1b8a3..ed8075c 100644
--- a/tests/Feature/InstallationTest.php
+++ b/tests/Feature/InstallationTest.php
@@ -49,4 +49,22 @@ class InstallationTest extends TestCase
 
         $this->assertDatabaseMissing('User', ['email' => 'another@example.test']);
     }
+
+    public function test_an_existing_installation_can_upgrade_its_recorded_initial_admin_once(): void
+    {
+        $legacyOwner = User::factory()->create(['isSuperAdmin' => false, 'passwordHash' => password_hash('password123', PASSWORD_BCRYPT)]);
+        ApplicationInstallation::create([
+            'id' => 'singleton',
+            'status' => 'INSTALLED',
+            'installedAt' => now(),
+            'initialAdminUserId' => $legacyOwner->id,
+        ]);
+
+        $this->post('/manager/sign-in', [
+            'email' => $legacyOwner->email,
+            'password' => 'password123',
+        ])->assertRedirect(route('manager.dashboard'));
+
+        $this->assertDatabaseHas('User', ['id' => $legacyOwner->id, 'isSuperAdmin' => 1]);
+    }
 }