Dont rename files, the Checkout wizard does not use a file as a template for the email body, so there is no email template file that you can rename to create the email message.
you will need to recreate the email receipt to send another one with slightly different wording.
if you want the second email to use a file for the email body, you will need to create one from scratch, add the cart display manager to it, select to use the email friendly design, and add any other content you want to show in the email.