1. There isn't any bulk update system right now. But you can update all your posts form association using this code below. Put that code on your theme functions.php, reload the page once and remove the code.

$form_id = 143;

$all_posts = new WP_Query( array('posts_per_page' => -1) );
if ( $all_posts->have_posts() ) {
    $posts = $all_posts->get_posts();

    foreach ($posts as $p) {
        update_post_meta( $p->ID, '_wpuf_form_id', $form_id );
    }
}

2. Make sure you are using the latest version (2.1.4).

So you can listen to posts when they inserts and set the form id like this:
[php]
function wpufe_auto_set_formid( $post_id, $post ) {
$form_id = 143;

if ( $post->post_type == ‘post' ) {
update_post_meta( $post_id, ‘_wpuf_form_id', $form_id );
}
}

add_action( ‘wp_insert_post', ‘wpufe_auto_set_formid', 10, 2 );
[/php]

Every new posts will have this form id automatically.

How did you tried? It should've worked!