diff -u b/core/modules/migrate_drupal/tests/src/Traits/CreateTestContentEntitiesTrait.php b/core/modules/migrate_drupal/tests/src/Traits/CreateTestContentEntitiesTrait.php
--- b/core/modules/migrate_drupal/tests/src/Traits/CreateTestContentEntitiesTrait.php
+++ b/core/modules/migrate_drupal/tests/src/Traits/CreateTestContentEntitiesTrait.php
@@ -57,59 +57,39 @@
   }
 
   /**
-   * Create several pieces of generic content, except for feeds.
+   * Create several pieces of generic content.
    */
   protected function createContent() {
-    $this->createBlockContent();
-    $this->createNodeContent();
-    $this->createFileContent();
-    $this->createMenuLinkContent();
-    $this->createTermContent();
-    $this->createUserContent();
-  }
-
-  /**
-   * Create an aggregator feed and item.
-   */
-  protected function createFeedContent() {
-    $feed = Feed::create([
-      'title' => 'feed',
-      'url' => 'http://www.example.com',
-    ]);
-    $feed->save();    // Create an aggregator feed.
+    // Create an aggregator feed.
     $feed = Feed::create([
       'title' => 'feed',
       'url' => 'http://www.example.com',
     ]);
     $feed->save();
+
+    // Create an aggregator feed item.
     $item = Item::create([
       'title' => 'feed item',
       'fid' => $feed->id(),
       'link' => 'http://www.example.com',
     ]);
     $item->save();
-  }
 
-  /**
-   * Create block content.
-   */
-  protected function createBlockContent() {
+    // Create a block content.
     $block = BlockContent::create([
       'info' => 'block',
       'type' => 'block',
     ]);
     $block->save();
-  }
 
-  /**
-   * Create a node and comment.
-   */
-  protected function createNodeContent() {
+    // Create a node.
     $node = Node::create([
       'type' => 'page',
       'title' => 'page',
     ]);
     $node->save();
+
+    // Create a comment.
     $comment = Comment::create([
       'comment_type' => 'comment',
       'field_name' => 'comment',
@@ -117,45 +97,29 @@
       'entity_id' => $node->id(),
     ]);
     $comment->save();
-  }
 
-  /**
-   *  Create a file.
-   */
-  protected function createFileContent() {
+    // Create a file.
     $file = File::create([
       'uri' => 'public://example.txt',
     ]);
     $file->save();
-  }
 
-  /**
-   * Create a menu link.
-   */
-  protected function createMenuLinkContent() {
+    // Create a menu link.
     $menu_link = MenuLinkContent::create([
       'title' => 'menu link',
       'link' => ['uri' => 'http://www.example.com'],
       'menu_name' => 'tools',
     ]);
     $menu_link->save();
-  }
 
-  /**
-   * Create a taxonomy term.
-   */
-  protected function createTermContent() {
+    // Create a taxonomy term.
     $term = Term::create([
       'name' => 'term',
       'vid' => 'term',
     ]);
     $term->save();
-  }
 
-  /**
-   * Create a user.
-   */
-  protected function createUserContent() {
+    // Create a user.
     $user = User::create([
       'uid' => 2,
       'name' => 'user',
@@ -166,2 +130,58 @@
 
+  /**
+   * Create several pieces of generic content.
+   */
+  protected function createContentPostUpgrade() {
+    // Create a block content.
+    $block = BlockContent::create([
+      'info' => 'Post upgrade block',
+      'type' => 'block',
+    ]);
+    $block->save();
+
+    // Create a node.
+    $node = Node::create([
+      'type' => 'page',
+      'title' => 'Post upgrade page',
+    ]);
+    $node->save();
+
+    // Create a comment.
+    $comment = Comment::create([
+      'comment_type' => 'comment',
+      'field_name' => 'comment',
+      'entity_type' => 'node',
+      'entity_id' => $node->id(),
+    ]);
+    $comment->save();
+
+    // Create a file.
+    $file = File::create([
+      'uri' => 'public://post_upgrade_example.txt',
+    ]);
+    $file->save();
+
+    // Create a menu link.
+    $menu_link = MenuLinkContent::create([
+      'title' => 'post upgrade menu link',
+      'link' => ['uri' => 'http://www.drupal.org'],
+      'menu_name' => 'tools',
+    ]);
+    $menu_link->save();
+
+    // Create a taxonomy term.
+    $term = Term::create([
+      'name' => 'post upgrade term',
+      'vid' => 'term',
+    ]);
+    $term->save();
+
+    // Create a user.
+    $user = User::create([
+      'uid' => 42,
+      'name' => 'universe',
+      'mail' => 'universe@example.com',
+    ]);
+    $user->save();
+  }
 }
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
@@ -40,6 +40,7 @@
     'content_translation',
     'migrate_drupal_ui',
     'telephone',
+    'aggregator',
     'book',
     'forum',
     'statistics',
@@ -201,6 +202,8 @@
     \Drupal::service('module_installer')->install(['aggregator']);
 
     // Test incremental migration.
+    $this->createContentPostUpgrade();
+
     // Need to update available and missing path lists.
     $this->drupalGet('/upgrade');
     $session->responseContains('An upgrade has already been performed on this site. To perform a new migration, create a clean and empty new install of Drupal 8. Rollbacks are not yet supported through the user interface.');
diff -u b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
@@ -83,16 +83,15 @@
     */
   protected function getEntityCountsIncremental() {
     $counts = $this->getEntityCounts();
-    $counts['aggregator_feed'] = 1;
-    $counts['aggregator_item'] = 1;
-    $counts['base_field_override'] = 38;
-    $counts['entity_form_display'] = 29;
+    $counts['block_content'] = 3;
+    $counts['comment'] = 7;
     $counts['entity_view_display'] = 53;
     $counts['entity_view_mode'] = 14;
-    $counts['field_config'] = 84;
-    $counts['field_storage_config'] = 58;
-    $counts['node_type'] = 13;
-    $counts['rdf_mapping'] = 7;
+    $counts['file'] = 9;
+    $counts['menu_link_content'] = 6;
+    $counts['node'] = 18;
+    $counts['taxonomy_term'] = 9;
+    $counts['user'] = 8;
     $counts['view'] = 16;
     return $counts;
   }
@@ -102,6 +101,7 @@
    */
   protected function getAvailablePaths() {
     return [
+      'aggregator',
       'block',
       'book',
       'comment',
@@ -146,7 +146,6 @@
    */
   protected function getMissingPaths() {
     return [
-      'aggregator',
       'date_api',
       'date_timezone',
       'event',
diff -u b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
@@ -39,6 +39,8 @@
    */
   protected function getEntityCounts() {
     return [
+      'aggregator_item' => 11,
+      'aggregator_feed' => 1,
       'block' => 25,
       'block_content' => 1,
       'block_content_type' => 1,
@@ -86,16 +88,15 @@
    */
   protected function getEntityCountsIncremental() {
     $counts = $this->getEntityCounts();
-    $counts['aggregator_feed'] = 1;
-    $counts['aggregator_item'] = 10;
-    $counts['base_field_override'] = 9;
-    $counts['entity_form_display'] = 17;
+    $counts['block_content'] = 2;
+    $counts['comment'] = 2;
     $counts['entity_view_display'] = 28;
     $counts['entity_view_mode'] = 14;
-    $counts['field_config'] = 61;
-    $counts['field_storage_config'] = 44;
-    $counts['rdf_mapping'] = 7;
-    $counts['taxonomy_term'] = 18;
+    $counts['file'] = 4;
+    $counts['menu_link_content'] = 9;
+    $counts['node'] = 6;
+    $counts['taxonomy_term'] = 19;
+    $counts['user'] = 5;
     $counts['view'] = 16;
     return $counts;
   }
@@ -105,6 +106,7 @@
    */
   protected function getAvailablePaths() {
     return [
+      'aggregator',
       'block',
       'comment',
       'contact',
@@ -148,7 +150,6 @@
    */
   protected function getMissingPaths() {
     return [
-      'aggregator',
       'blog',
       'book',
       'color',