diff -u b/modules/system/system.install b/modules/system/system.install
--- b/modules/system/system.install
+++ b/modules/system/system.install
@@ -3173,22 +3173,14 @@
   );
   db_drop_primary_key('sessions');
   db_change_field('sessions', 'sid', 'sid', $spec, array('primary key' => array('sid', 'ssid')));
-  $sessions = db_query('SELECT sid, ssid FROM {sessions}');
-  while ($session = $sessions->fetchAssoc()) {
-    $query = db_update('sessions');
-    $fields = array();
-    if (!empty($session['sid'])) {
-      $fields['sid'] = drupal_hash_base64($session['sid']);
-      $query->condition('sid', $session['sid']);
-    }
-    if (!empty($session['ssid'])) {
-      $fields['ssid'] = drupal_hash_base64($session['ssid']);
-      $query->condition('ssid', $session['ssid']);
-    }
-    db_update('sessions')
-      ->fields($fields)
-      ->execute();
-  }
+
+  // Delete all the existing old sessions.
+  db_truncate('sessions')->execute();
+  // Force to write again the current session to have it hashed.
+  drupal_save_session(TRUE);
+  _drupal_session_write(session_id(), '');
+  drupal_save_session(FALSE);
+
 }
 
 /**