This can be done by customizing the form-billing.php template, just move this part:
<?php if (!is_user_logged_in() && get_option('woocommerce_enable_signup_and_login_from_checkout')=="yes") : ?>
//more code..
<?php endif; ?>
right before this line:
<?php do_action('woocommerce_before_checkout_billing_form', $checkout); ?>