John Davidson

add_action('woocommerce_checkout_fields','customization_readonly_billing_fields',10,1);

function customization_readonly_billing_fields($checkout_fields){

    $current_user = wp_get_current_user();;

    $user_id = $current_user->ID;

    foreach ( $checkout_fields['billing'] as $key => $field ){

        if($key == 'billing_address_1' || $key == 'billing_address_2'){

            $key_value = get_user_meta($user_id, $key, true);

            if( strlen($key_value)>0){

                $checkout_fields['billing'][$key]['custom_attributes'] = array('readonly'=>'readonly');

            }

        }

    }

    return $checkout_fields;

}

---

My Answer:
name field as well:

php
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

function custom_override_checkout_fields( $fields ) {
$fields['billing']['billing_first_name']['custom_attributes'] = array( 'readonly' => 'readonly' );
$fields['billing']['billing_last_name']['custom_attributes'] = array( 'readonly' => 'readonly' );
$fields['billing']['billing_address_1']['custom_attributes'] = array( 'readonly' => 'readonly' );
$fields['billing']['billing_address_2']['custom_attributes'] = array( 'readonly' => 'readonly' );
$fields['billing']['billing_city']['custom_attributes'] = array( 'readonly' => 'readonly' );
$fields['billing']['billing_postcode']['custom_attributes'] = array( 'readonly' => 'readonly' );
$fields['billing']['billing_country']['custom_attributes'] = array( 'readonly' => 'readonly' );
$fields['billing']['billing_state']['custom_attributes'] = array( 'readonly' => 'readonly' );

return $fields;
}


This code will make the first name, last name, and address fields in the billing section read-only on the WooCommerce checkout page.