add_action( 'woocommerce_loaded', 'wc_custom_loaded' );

function wc_custom_loaded() {
	$old_statuses = array(
		'failed',
		//uncomment any of the below statuses to include those statuses
		//'pending',
		//'processing',
		//'on-hold',
		//'cancelled',
		//'refunded'
	);
	foreach ( $old_statuses as $old_status ) {
		add_action( 'woocommerce_order_status_' . $old_status . '_to_completed', 'wc_custom_change_posted_date', 99, 1 );
	}
}

function wc_custom_change_posted_date( $order_id ) {
	$order = wc_get_order( $order_id );
	$args = array(
		'post_id' => $order_id,
		//wp_insert_post (called by wp_update_post) will set the date to "now" if `post_date` is empty, likewise with `post_date_gmt`
		'post_date' => '',
		'post_date_gmt' => '',
	);
	wp_update_post( $args );
}