When it comes to “editing” stuff, chances are WooCommerce gives us a “filter” we can use without overriding core. The more filters, the more customization we can do. In this case, I wanted to change the “Returning Customer?” message on my own checkout.

PHP Snippet: Rename “Returning Customer?” @ WooCommerce Checkout
/**
* @snippet Rename "Returning Customer?" @ Checkout Page - WooCommerce
* @how-to Get CustomizeWoo.com FREE
* @sourcecode https://businessbloomer.com/?p=21719
* @author Rodolfo Melogli
* @compatible WC 3.5.4
* @donate $9 https://businessbloomer.com/bloomer-armada/
*/
add_filter( 'woocommerce_checkout_login_message', 'bbloomer_return_customer_message' );
function bbloomer_return_customer_message() {
return 'Have you shopped with us before?';
}