Delayed Autoresponders

Can I add a new delayed autoresonder after registration has begun? For example: 

I create a registration form for an event that is occurring on August 14. 20 people register for the event before August 11. I want to communicate new details to those who have already registered. If on August 12 I create an autoresponder and schedule it to go out that day, will those who have already registered receive it?

You can always create new autoresponders. This new notification will only affect the new forms that are submitted. If you wish to inform users that have already send a form, you will need to send them an email manually.