Update preorder date programmatically for Variable products

functions.php

add_action('wp_head','bp_preorder_script');

function bp_preorder_script() {
	$args = array(
    'post_type'      => 'product_variation',
    'posts_per_page' => -1,

);
$query = new WP_Query( $args );
	
foreach ( $query->posts as $key => $vp ) {

	update_post_meta( $vp->ID, '_is_pre_order', 'yes' );
  update_post_meta( $vp->ID, '_pre_order_date', '2021-04-15' ); // change date with same format
}
	
}