Great question! You can absolutely change the signup form sent with each mailing and you will edit the forms on the Review and Send page. After you've entered your recipient list and subject line, you can change the signup form attached to the mailing in the Other Options section:
Hi Emily, I have the same question. However, when I go into the Review and Send screen from creating your template, I don't have the same screen above or an option to choose a sign-up form. Did this change within the year?