Steps to modify FOSCommentBundle form and modify it based on thread id

CommentFormSubscriber.php

<?php
namespace CommentBundle\Event;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use CommentBundle\Event\CommentFormEvent;

class CommentFormSubscriber implements EventSubscriberInterface
{
    public function form(CommentFormEvent $event)
    {
        if($event->getId() == 'test')
            $event->getForm()->remove('rating');
    }

    public static function getSubscribedEvents()
    {
        return array(
            'fos_comment.comment.form' => array('form')
        );
    }
}

CommentType.php

class CommentType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('body', 'textarea');
        $builder->add('rating', 'choice', array(
            'choices'   => array(
                '5' => '5',
                '4' => '4',
                '3' => '3',
                '2' => '2',
                '1' => '1',                
            ),
            'multiple'  => false,
            'expanded' => true
        ));
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        parent::setDefaultOptions($resolver);
        $resolver->setDefaults(array(
            'data_class' => 'App\Entity\Comment',
        ));
    }

    public function getName()
    {
        return "fos_comment_comment";
    }
}

routing.yml

fos_comment_thread_api:
    type: rest
    resource: CommentBundle\Controller\ThreadController
    name_prefix: fos_comment_

services.yml

services:
    fos_comment.form_type.comment.default:
        class: App\Form\CommentType
        tags: [{ name: form.type, alias: fos_comment_comment }]

ThreadController.php

<?php

namespace CommentBundle\Controller;

use FOS\CommentBundle\Model\ThreadInterface;
use FOS\RestBundle\View\View;
use Symfony\Bundle\FrameworkBundle\Templating\TemplateReference;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

use CommentBundle\Event\CommentFormEvent;
use CommentBundle\Event\CommentFormSubscriber;

use FOS\CommentBundle\Controller\ThreadController as BaseController;

class ThreadController extends BaseController
{
    /**
     * Presents the form to use to create a new Comment for a Thread.
     *
     * @param string $id
     *
     * @return View
     */
    public function newThreadCommentsAction($id)
    {
        $thread = $this->container->get('fos_comment.manager.thread')->findThreadById($id);
        if (!$thread) {
            throw new NotFoundHttpException(sprintf('Thread with identifier of "%s" does not exist', $id));
        }

        $comment = $this->container->get('fos_comment.manager.comment')->createComment($thread);

        $parent = $this->getValidCommentParent($thread, $this->getRequest()->query->get('parentId'));

        $form = $this->container->get('fos_comment.form_factory.comment')->createForm();

        $dispatcher = $this->container->get('event_dispatcher');
        $dispatcher->addSubscriber(new CommentFormSubscriber());
        $dispatcher->dispatch('fos_comment.comment.form', new CommentFormEvent($form, $id));

        $form->setData($comment);

        $view = View::create()
            ->setData(array(
                'form' => $form->createView(),
                'first' => 0 === $thread->getNumComments(),
                'thread' => $thread,
                'parent' => $parent,
                'id' => $id,
            ))
            ->setTemplate(new TemplateReference('FOSCommentBundle', 'Thread', 'comment_new'));

        return $this->getViewHandler()->handle($view);
    }

    /**
     * Checks if a comment belongs to a thread. Returns the comment if it does.
     *
     * @param ThreadInterface $thread    Thread object
     * @param mixed           $commentId Id of the comment.
     *
     * @return CommentInterface|null The comment.
     */
    protected function getValidCommentParent(ThreadInterface $thread, $commentId)
    {
    {
        return parent::getValidCommentParent($thread, $commentId);
    }

    /**
     * @return \FOS\RestBundle\View\ViewHandler
     */
    protected function getViewHandler()
    {
        return $this->container->get('fos_rest.view_handler');
    }

}