diff --git a/core/modules/migrate/src/Entity/Migration.php b/core/modules/migrate/src/Entity/Migration.php
index 9fb93b7..102098a 100644
--- a/core/modules/migrate/src/Entity/Migration.php
+++ b/core/modules/migrate/src/Entity/Migration.php
@@ -41,21 +41,21 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var string
    */
-  public $id;
+  protected $id;
 
   /**
    * The human-readable label for the migration.
    *
    * @var string
    */
-  public $label;
+  protected $label;
 
   /**
    * The plugin ID for the row.
    *
    * @var string
    */
-  public $row;
+  protected $row;
 
   /**
    * The source configuration, with at least a 'plugin' key.
@@ -64,7 +64,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var array
    */
-  public $source;
+  protected $source;
 
   /**
    * The source plugin.
@@ -78,14 +78,14 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var array
    */
-  public $process;
+  protected $process;
 
   /**
    * The configuration describing the load plugins.
    *
    * @var array
    */
-  public $load;
+  protected $load;
 
   /**
    * The cached process plugins.
@@ -101,7 +101,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var array
    */
-  public $destination;
+  protected $destination;
 
   /**
    * The destination plugin.
@@ -117,7 +117,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var string
    */
-  public $idMap = array();
+  protected $idMap = array();
 
   /**
    * The identifier map.
@@ -134,7 +134,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var array
    */
-  public $sourceIds = array();
+  protected $sourceIds = array();
 
   /**
    * The destination identifiers.
@@ -144,14 +144,14 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var array
    */
-  public $destinationIds = FALSE;
+  protected $destinationIds = FALSE;
 
   /**
    * Information on the high water mark.
    *
    * @var array
    */
-  public $highWaterProperty;
+  protected $highWaterProperty;
 
   /**
    * Indicate whether the primary system of record for this migration is the
@@ -163,7 +163,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var string
    */
-  public $systemOfRecord = self::SOURCE;
+  protected $systemOfRecord = self::SOURCE;
 
   /**
    * Specify value of source_row_status for current map row. Usually set by
@@ -171,7 +171,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var int
    */
-  public $sourceRowStatus = MigrateIdMapInterface::STATUS_IMPORTED;
+  protected $sourceRowStatus = MigrateIdMapInterface::STATUS_IMPORTED;
 
   /**
    * @var \Drupal\Core\KeyValueStore\KeyValueStoreInterface
@@ -181,7 +181,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
   /**
    * @var bool
    */
-  public $trackLastImported = FALSE;
+  protected $trackLastImported = FALSE;
 
   /**
    * These migrations must be already executed before this migration can run.
@@ -195,7 +195,7 @@ class Migration extends ConfigEntityBase implements MigrationInterface, Requirem
    *
    * @var array
    */
-  public $migration_dependencies = array();
+  protected $migration_dependencies = array();
 
   /**
    * The entity manager.
diff --git a/core/modules/migrate_drupal/src/Plugin/migrate/load/LoadEntity.php b/core/modules/migrate_drupal/src/Plugin/migrate/load/LoadEntity.php
index adbd919..adc6ff6 100644
--- a/core/modules/migrate_drupal/src/Plugin/migrate/load/LoadEntity.php
+++ b/core/modules/migrate_drupal/src/Plugin/migrate/load/LoadEntity.php
@@ -92,7 +92,8 @@ public function loadMultiple(EntityStorageInterface $storage, array $sub_ids = N
             // Specifically process the link field until core is fixed.
             // @see https://www.drupal.org/node/2235457
             if ($data['type'] == 'link') {
-              $migration->process[$field_name] = [
+              $process = $migration->get('process');
+              $process[$field_name] = [
                 'plugin' => 'd6_cck_link',
                 'source' => [
                   $field_name,
@@ -100,15 +101,18 @@ public function loadMultiple(EntityStorageInterface $storage, array $sub_ids = N
                   $field_name . '_attributes',
                 ],
               ];
+              $migration->set('process', $process);
             }
             else {
-              $migration->process[$field_name] = $field_name;
+              $process = $migration->get('process');
+              $process[$field_name] = $field_name;
+              $migration->set('process', $process);
             }
           }
         }
         else {
           $fields = array_keys($migration->getSourcePlugin()->fields());
-          $migration->process += array_combine($fields, $fields);
+          $migration->set('process', $migration->get('process') + array_combine($fields, $fields));
         }
         $migrations[$migration->id()] = $migration;
       }
diff --git a/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php b/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php
index 48645c1..b613f19 100644
--- a/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php
+++ b/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php
@@ -34,7 +34,9 @@ protected function setUp() {
     );
     /** @var \Drupal\migrate\entity\Migration $migration */
     $migration = entity_load('migration', 'd6_file');
-    $migration->source['conf_path'] = 'core/modules/simpletest';
+    $source = $migration->get('source');
+    $source['conf_path'] = 'core/modules/simpletest';
+    $migration->set('source', $source);
     $this->prepare($migration, $dumps);
     $executable = new MigrateExecutable($migration, $this);
     $executable->import();
diff --git a/core/modules/migrate_drupal/src/Tests/d6/MigrateUserPictureFileTest.php b/core/modules/migrate_drupal/src/Tests/d6/MigrateUserPictureFileTest.php
index ff204f7..16e8e1b 100644
--- a/core/modules/migrate_drupal/src/Tests/d6/MigrateUserPictureFileTest.php
+++ b/core/modules/migrate_drupal/src/Tests/d6/MigrateUserPictureFileTest.php
@@ -34,7 +34,9 @@ protected function setUp() {
     );
     /** @var \Drupal\migrate\entity\Migration $migration */
     $migration = entity_load('migration', 'd6_user_picture_file');
-    $migration->source['conf_path'] = 'core/modules/simpletest';
+    $source = $migration->get('source');
+    $source['conf_path'] = 'core/modules/simpletest';
+    $migration->set('source', $source);
     $this->prepare($migration, $dumps);
     $executable = new MigrateExecutable($migration, $this);
     $executable->import();