Hey @laughmasters,
This is possible now as of version 4.2 of Event Tickets Plus. (Please note that you’d need to also have The Events Calendar and Event Tickets at version 4.2, as well.)
As of 4.2, there is an option to require users to log-in to your site before being able to buy tickets or make RSVPs. If this option is enabled, then users can log back onto your site before the event they RSVPed to, and change their RSVP status from “Going” to “Not Going.”
Without that “require users to log-in” option being set to “true”, this is not possible. 🙁
You can learn more here in the “Letting Attendees Edit Their Information” section here ? https://theeventscalendar.com/knowledgebase/managing-your-orders-and-attendees/#editing-attendee-info
I hope this helps!
George