Check if user_picture value is changed or compare with old value
Perhaps you should use hook_entity_presave that keeps the old (original) value 

function MODULENAME_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {

  if($entity-&gt;getEntityTypeId() == "file"){

    $filename = $entity-&gt;getFilename();

    $file_old = $entity-&gt;original;

    if($file_old != null){
      $filename_old = $file_old-&gt;getFilename();

      if($filename_old !== $filename){
        $entity-&gt;setFilename('newfile.jpg');
      }
    }
  }
}


The function above changes the filename if its not the same as old filename.

Depending on your use case you might want to apply the above logic when user is presaved

function MODULENAME_entity_presave(Drupal\Core\Entity\EntityInterface $entity)
  if($entity-&gt;getEntityTypeId() == "user"){

    // get specific field by field machine name
    $file = $entity-&gt;get('field_file');

    // or get the user_picture field
    $picture = $entity-&gt;get('user_picture');

    $user_old = $entity-&gt;original;
    // drill down in the old user object to see values changes
    // and do appropriate changes
  }
}