Change the class of an object at runtime

// existing code --------------------
class JParameter {
    function render() {
        // return HTML with tables
    }
    // of course, there's a lot more functions here
}

// my magical class -----------------
class MyParameter extends JParameter {
    function render() {
        // return HTML which doesn't suck
    }
}

// my code --------------------------
$this->params->render();    // returns tables
$this->params = (MyParameter) $this->params;  // miracle occurs here?
$this->params->render();    // returns nice html

class MyJParameter {
    private $jparm;
    function __construct( JParameter $jparm ) {
        $this->jparm = $jparm;
    }
    function render() {
        /* your code here */
    }
    function __get( $var ) {
        if( isset( $this->$jparm->$var ) {
            return $this->$jparm->$var;
        }
        return false;
    }
    function __set( $var, $val ) {
        /* similar to __get */
    }
    function __call( $method, $arguments ) {
        if( method_exists( $this->jparm, $method ) {
           return call_user_func_array( array( $this->jparm, $method ), $arguments );
        }
        return false;
    }
}