In Drupal 7 a common development pattern to update form elements during an AJAX callback was to use code that did this:

function my_d7_ajax_callback(&$form, &$form_state) {
//Where my_field is a textfield with a div wrapper of my_field_wrapper
$form['my_field']['#value'] = 'new value';
$commands[] = ajax_command_replace('#my_field_wrapper', render($form['my_field']));
return array('#type' => 'ajax', '#commands' => $commands);
}

In Drupal 8 this pattern looks like:

public function my_d8_ajax_callback(array &$form, FormStateInterface $form_state) {
//Where my_field is a textfield with no explicit wrapper.
$form['my_field']['#value'] = "new value";
$response = new AjaxResponse();
$response->addCommand(new ReplaceCommand('.form-item-my-field', $form['my_field']));
return $response;
}

Important points:

• D7 ajax_command_replace is ReplaceCommand in D8
• There is no need to call the render function, D8 automatically renders the specified form item.