Do not know who originally changes this
<?php do_action('woocommerce_email_header', $email_heading); ?>
-<p><?php _e( "Your order has been received and is now being processed. Your order details are shown below for your reference:", 'woocommerce' ); ?></p>
+<p><?php _e( "Thank you for your order! We have received the following order details, and will be processing your order shortly.", 'woocommerce' ); ?></p>
<?php do_action( 'woocommerce_email_before_order_table', $order, $sent_to_admin, $plain_text ); ?>