When you create a Public Event, but meant that it should be Private, you can't change it once you save it and vice versa.   Even in Draft mode.  This request is from one of our admins who creates a lot of events:

• I've made this mistake a few times (my mind wanders sometimes 😲) and had to cancel the event and recreate one. It would have been easier if the "Type" field could have been modified after creating the event.