Display custom type posts instead another custom type posts

I develope my WP site so I have 2 custom post types "cases" and "newcases". The "newcases" posts have new design and content. So I have more than 500 "cases" posts and only 20 "newcases".
I need to replace 20 first "cases" with those 20 "newcases" so my permalinks stay as it was (/%category%/%postname%/).
I thought it was simple task and tried like everything(code is below.
Now i get stucked, have you any suggestions?

global $post;
$post_id = $post->ID;

if( $post_id == 27 ) {
   $my_query = new WP_Query('p=29');
   } else {
   $my_query = new WP_Query('p='.$post_id);
}

function custom_pages_services( $query ) {
    global $post;
    if ( ! is_admin() && $query->is_main_query() ) {
        $query->set( 'p', 29 );
        return;
    }
}
add_action( 'pre_get_posts', 'custom_pages_services', 1 );