SQL / wp_update_post: change post custom field to CPT post taxonomy

You’ve got the right idea but you are using some of the functions and function arguments incorrectly.

// get all post IDs
$post_ids = get_posts(
  array(
    'post_type' => 'post',
    'post_status' => 'publish',
    'numberposts'   => -1, // get all posts.
    'fields'     => 'ids', // Only get post IDs
  )
);

// info: custom field 'actor' = 'Tom Hanks'
$customfield_value = get_post_meta($post_ids, 'actor', true);
// change the custom field values to slug format:  'Tom Hanks' > 'tom-hanks'
$customfield_value = sanitize_title_with_dashes($customfield_value);

// Update all posts
foreach ($post_ids as $id) {
  $post = array(
    'ID' => $id,      // select all posts - not sure if that works with the array from $post_ids.
    'tax_input'      => array( 
      'actor' => array( 
        $customfield_value
      ) 
    )
  );
  // Update the post into the database
  wp_update_post( $post );

  // delete the old post meta after we moved it to the taxononmy
  delete_post_meta($id, 'actor'); 
}

That is untested. Do not run that on a production database without first testing it on a copy first. Be very, very careful especially with deleting the post meta. That is not reversible.

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