show-related-product-top-of-out-of-stock-products.php

<?php

/**
 * Add action to top of single product
 */
add_action( 'woocommerce_before_single_product', 'ywp_related_first_in_out_of_stocks', 5 );
function ywp_related_first_in_out_of_stocks() {
	global $product;

    if ( ! $product->is_in_stock() ) {
		woocommerce_output_related_products();
		
		// Remove default related products
		remove_action( 'woocommerce_after_single_product_summary', 'woocommerce_output_related_products', 20 );
    }
}

/**
 * The code goes in your theme/child theme functions.php 
 */