It isn't possible to make chart changes that only affect one of the events associated with it.
However, there are two possible options to handle this.
Option A: Duplicate the chart, then create an event.
By duplicating the chart, you can have an independent copy where to make changes for a new event created within it.
Option B: Detach existing event from a chart.
If your event is already live, you might want to detach it instead, from the Event details page.
This will duplicate the chart and move this particular event onto the new chart. That way, all the changes you've made on the event level, as well as bookings, will transfer over.