diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
index 734cab05a6..cf17246ea9 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
@@ -296,11 +296,11 @@ protected function getDefinitionFromEntity($key) {
    * {@inheritdoc}
    */
   public function getHighestId() {
-    $query = $this->storage->getQuery()
+    $values = $this->storage->getQuery()
       ->sort($this->getKey('id'), 'DESC')
-      ->range(0, 1);
-    $result = $query->execute();
-    return (int) reset($result);
+      ->range(0, 1)
+      ->execute();
+    return (int) current($values);
   }
 
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
index a3c1e6a853..290c1e1b5a 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
@@ -97,11 +97,14 @@ public function getIds() {
    * {@inheritdoc}
    */
   public function getHighestId() {
-    $query = $this->storage->getQuery()
+    $values = $this->storage->getQuery()
+      ->allRevisions()
       ->sort($this->getKey('revision'), 'DESC')
-      ->range(0, 1);
-    $result = $query->execute();
-    return (int) reset($result);
+      ->range(0, 1)
+      ->execute();
+    // The array keys are the revision IDs.
+    // The array contains only one entry, so we can use key().
+    return (int) key($values);
   }
 
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
index be0428162c..8bc17f19f5 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -990,7 +990,7 @@ function (array $id) {
     }
 
     // Return the highest of all the mapped IDs.
-    return max($ids);
+    return (int) max($ids);
   }
 
 }
diff --git a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
index 08a0498d21..9c05b6232c 100644
--- a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
@@ -507,7 +507,7 @@ public function buildIdConflictForm(array &$form, FormStateInterface $form_state
 
     $form['warning'] = [
       '#type' => 'markup',
-      '#markup' => '<h3>' . $this->t('Content may be overwritten') . '</h3>',
+      '#markup' => '<h3>' . $this->t('WARNING: Content may be overwritten') . '</h3>',
     ];
     $form = parent::buildForm($form, $form_state);
     $form['actions']['submit']['#submit'] = ['::submitConfirmIdConflictForm'];
@@ -536,7 +536,7 @@ public function buildIdConflictForm(array &$form, FormStateInterface $form_state
    *   The form structure.
    */
   protected function conflictsForm(array &$form, FormStateInterface $form_state, array $conflicts) {
-    $form['warning']['#markup'] .= '<p>' . $this->t('Upgrades work on clean and empty new installs of Drupal 8. However, it looks like you have content in your site that is unknown to the migrate system; perhaps manually added. These new entities <strong>may be overwritten</strong> if you run this upgrade. For more information, see the online handbook entry for <a target="_blank" href=":id-conflicts-handbook">handling migration conflicts</a>.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
+    $form['warning']['#markup'] .= '<p>' . $this->t('The upgrade should be performed on a clean Drupal 8 installation. It looks like you have content on your site which <strong>may be overwritten</strong> if you continue to run the upgrade. For more information, refer to the <a target="_blank" href=":id-conflicts-handbook">upgrade handbook</a>.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
 
     $form['conflicts'] = [
       '#title' => $this->t('The conflicting content are of the following types:'),
@@ -587,7 +587,7 @@ protected function formatConflicts(array $conflicts) {
    *   The form structure.
    */
   protected function i18nWarningForm(array &$form, FormStateInterface $form_state, array $conflicts) {
-    $form['warning']['#markup'] .= '<p>' . $this->t('It looks like you are migrating translated content. Be extra cautious and make sure you aren\'t introducting any conflicts. For more on the subject, see the online handbook entry for <a target="_blank" href=":id-conflicts-handbook">handling migration conflicts</a>.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
+    $form['warning']['#markup'] .= '<p>' . $this->t('It looks like you are migrating translated content. The possible ID conflicts for translations are not automatically detected in the current version of Drupal. Refer to the <a target="_blank" href=":id-conflicts-handbook">upgrade handbook</a> for instructions on how to avoid ID conflicts with translated content.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
 
     $form['i18n'] = [
       '#title' => $this->t('The translated content are of the following types:'),