All the details for the registrations category.
Generated a new discount code on a registration template
Updates an existing discount code on a registration template
Be careful using the "Update Past Registrations" option- remember that people will have been told the total amounts that is due (and received a confirmation email proving it), and that if you set this to "Yes", that you're changing that amount on them in mid-stream and with no (automatic) notice. Personal contact is going to be key, if you find you have to do this.