WordPress - Loop through selected posts

<?php
  // select some posts and loop through them to do something
  // code goes in functions.php for your child theme

  // see https://codex.wordpress.org/Template_Tags/get_posts for all possible arguments
  $args = array(
   'post_type' => 'post',
   'post_status' => 'any',
   'numberposts' => -1 );

  $selected_posts = get_posts( $args );

  foreach( $selected_posts as $post ) {
    setup_postdata($post);
    $post_id = get_the_ID();
    set_post_type( $post_id, 'product' );
    add_post_meta( $post_id, '_price', ' >>> PRICE <<< ', true);
    add_post_meta( $post_id, '_regular_price', ' >>> PRICE <<< ', true);
    add_post_meta( $post_id, '_visibility', 'visible', true);
    add_post_meta( $post_id, '_stock_status', 'instock', true);
  }

  wp_reset_postdata();