Alter the Registered Post Type Args for Any Post Type

registered_post_type.php

<?php
/***************************************************************
* Function bct_make_whistles_public()
* Sets the whistles post type to be public
***************************************************************/
function bct_make_whistles_public( $post_type, $args ) {
	
	/* only proceed for the whistles post type */
	if ( 'whistle' != $post_type )
		return;
	
	global $wp_post_types;
	
	/* set post type to public */
	$args->public = true;
	
	/* modify post type object to implement the changes */
	$wp_post_types[ $post_type ] = $args;
	
}

add_action( 'registered_post_type', 'bct_make_whistles_public', 10, 2 );