Symfony can changing the 'secret' parameter break anything?
It does not break anything. Changing the value secret parameter from time to time is even considered a good practice, suggested by the official documentation:
http://symfony.com/doc/current/reference/configuration/framework.html#secret
The only thing to be aware of is:
However, keep in mind that changing this value will invalidate all signed URIs and Remember Me cookies. That's why, after changing this value, you should regenerate the application cache and log out all the application users.