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<?php 

$query = new WP_Query([
    'post_type' => 'post',
    'cat' => 4,
    'per_page' => -1,
]);

while ($query->have_posts()) {
    $query->the_post();
    $post_id = get_the_ID();
    the_title();
    the_category();
    echo '<br>';

    $wpdb->query("UPDATE $wpdb->posts SET post_type = 'video' WHERE ID = $post_id LIMIT 1");
}