As long as your list of attendees is empty you can change from a ticket event to a RSVP-event. Remember that you can also set up ticket batches that are free. If you have multiple dates or different options at the sign up you need to have a ticket event even if it will be free.
Change a ticket event to RSVP
Follow these steps to set your event to RSVP instead of ticket:
Click on `Settings` in the menu
Click on `Setup`
Scroll down to the tickets section
Check the box `Free event`