diff -u b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -111,6 +111,29 @@
    * Executes all steps of migrations upgrade.
    */
   public function testMigrateUpgrade() {
+    $driver = $this->sourceDatabase->getConnectionOptions()['driver'];
+    $edits = $this->getUiEditsArray();
+
+    // Ensure submitting the form with invalid database credentials gives us a
+    // nice warning.
+    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
+    $this->assertText('Resolve the issue below to continue the upgrade.');
+
+    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertResponse(200);
+    $this->assertText('Are you sure?');
+    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
+    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+
+    $this->runPostMigrationTests();
+  }
+
+  /**
+   * Build a re-usable form $edits array for the UI.
+   *
+   * @return array $edits
+   */
+  protected function getUiEditsArray() {
     $connection_options = $this->sourceDatabase->getConnectionOptions();
     $this->drupalGet('/upgrade');
     $this->assertText('Upgrade a site by importing it into a clean and empty new install of Drupal 8. You will lose any existing configuration once you import your site into it. See the online documentation for Drupal site upgrades for more detailed information.');
@@ -137,20 +160,14 @@
     if (count($drivers) !== 1) {
       $edit['driver'] = $driver;
     }
-    $edits = $this->translatePostValues($edit);
-
-    // Ensure submitting the form with invalid database credentials gives us a
-    // nice warning.
-    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
-    $this->assertText('Resolve the issue below to continue the upgrade.');
-
-    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
-    $this->assertResponse(200);
-    $this->assertText('Are you sure?');
-    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
-    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
-    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+    return $this->translatePostValues($edit);
+  }
 
+  /**
+   * Runs all post migration tests.
+   */
+  protected function runPostMigrationTests() {
+    $version = $this->getLegacyDrupalVersion($this->sourceDatabase);
     // Have to reset all the statics after migration to ensure entities are
     // loadable.
     $this->resetAll();
@@ -183,12 +200,10 @@
           $this->fail($message);
         }
         else {
-          $this->pass($message);
+          $this->assertTrue(TRUE, $message);
         }
       }
     }
-    \Drupal::service('module_installer')->install(['forum']);
-    \Drupal::service('module_installer')->install(['book']);
   }
 
   /**
only in patch2:
unchanged:
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
@@ -15,6 +15,21 @@
 class MigrateUpgrade6Test extends MigrateUpgradeTestBase {
 
   /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = [
+    'language',
+    'content_translation',
+    'migrate_drupal_ui',
+    'telephone',
+    'aggregator',
+    'book',
+    'statistics',
+  ];
+
+  /**
    * {@inheritdoc}
    */
   protected function setUp() {
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgradeConflicts6Test.php
@@ -0,0 +1,55 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal_ui\Functional\d6;
+
+use Drupal\Tests\migrate_drupal_ui\Functional\MigrateUpgradeTestBase;
+use Drupal\user\Entity\User;
+
+/**
+ * Tests Drupal 6 upgrade using the migrate UI and has ID conflicts.
+ *
+ * The test method is provided by the MigrateUpgradeTestBase class.
+ *
+ * @group migrate_drupal_ui
+ */
+class MigrateUpgradeConflicts6Test extends MigrateUpgrade6Test {
+
+  /**
+   * Use the Minimal profile to limit the number of modules enabled.
+   */
+  protected $profile = 'minimal';
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   *
+   * Forum module adds content, resulting in content id conflicts.
+   */
+  public static $modules = [
+    'forum',
+  ];
+
+  /**
+   * Executes all steps of migrations upgrade.
+   */
+  public function testMigrateUpgrade() {
+    $driver = $this->sourceDatabase->getConnectionOptions()['driver'];
+    $edits = $this->getUiEditsArray();
+
+    // Ensure submitting the form with invalid database credentials gives us a
+    // nice warning.
+    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
+    $this->assertText('Resolve the issue below to continue the upgrade.');
+
+    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertResponse(200);
+    $this->assertText('Are you sure?');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
+    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
+    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+
+    $this->runPostMigrationTests();
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgradeConflicts7Test.php
@@ -0,0 +1,55 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal_ui\Functional\d7;
+
+use Drupal\Tests\migrate_drupal_ui\Functional\MigrateUpgradeTestBase;
+use Drupal\user\Entity\User;
+
+/**
+ * Tests Drupal 7 upgrade using the migrate UI and has ID conflicts.
+ *
+ * The test method is provided by the MigrateUpgradeTestBase class.
+ *
+ * @group migrate_drupal_ui
+ */
+class MigrateUpgradeConflicts7Test extends MigrateUpgrade7Test {
+
+  /**
+   * Use the Minimal profile to limit the number of modules enabled.
+   */
+  protected $profile = 'minimal';
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   *
+   * Forum module adds content, resulting in content id conflicts.
+   */
+  public static $modules = [
+    'forum',
+  ];
+
+  /**
+   * Executes all steps of migrations upgrade.
+   */
+  public function testMigrateUpgrade() {
+    $driver = $this->sourceDatabase->getConnectionOptions()['driver'];
+    $edits = $this->getUiEditsArray();
+
+    // Ensure submitting the form with invalid database credentials gives us a
+    // nice warning.
+    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
+    $this->assertText('Resolve the issue below to continue the upgrade.');
+
+    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertResponse(200);
+    $this->assertText('Are you sure?');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
+    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
+    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+
+    $this->runPostMigrationTests();
+  }
+
+}