diff -u b/core/modules/migrate_drupal_ui/src/MigrateUpgradeImportBatch.php b/core/modules/migrate_drupal_ui/src/MigrateUpgradeImportBatch.php
--- b/core/modules/migrate_drupal_ui/src/MigrateUpgradeImportBatch.php
+++ b/core/modules/migrate_drupal_ui/src/MigrateUpgradeImportBatch.php
@@ -65,9 +65,8 @@
     if ($destination['plugin'] === 'entity:file') {
       // Make sure we have a single trailing slash.
       $source_base_path = rtrim($config['source_base_path'], '/') . '/';
-      $source = $migration->getSourceConfiguration();
-      $source['constants']['source_base_path'] = $source_base_path;
-      $migration->set('source', $source);
+      $destination['source_base_path'] = $source_base_path;
+      $migration->set('destination', $destination);
     }
 
     if ($migration) {