Modify events in calendar to become recurring - calendar

Question

I am trying to create a Google Script that goes through each event within a time range and modifies the events to a yearly recurring event. Currently, I mistakenly put in a bunch of birthdays into a calendar, but did not set them as recurring. Instead of going through each one manually, I wanted to create a script that sets them to a yearly recurrence. I'm getting stuck on line 12.
NOTE: Currently, I simply have my code searching for one specific event called "john bday" but would take out the "if" statement once I get it to modify the existing event correctly. Please advise and thank you.
function myFunction() {
var fromDate = new Date(2019,0,1,0,0,0); //This is January 1, 2019
var toDate = new Date(2019,2,31,0,0,0); //This is March 31, 2019
var calname = "testing calendar";
var findtitle = "john bday";
var calendar = CalendarApp.getCalendarsByName(calname)[0];
var events = calendar.getEvents(fromDate,toDate);
for (var i=0; i<events.length;i++) {
var ev = events[i];
if (ev.getTitle()==findtitle) {
CalendarApp.newRecurrence().addYearlyRule();
}
}
}

Answer 1

I was able to get it to work by copying the event into a new series and then deleting the original event. Thanks to all of those who took a look at this.
function myFunction() {
var fromDate = new Date(2018,6,1,0,0,0); //This is July 1, 2018
var toDate = new Date(2019,0,31,0,0,0); //This is January 31, 2019
var calname = "testing calendar";
var calendar = CalendarApp.getCalendarsByName(calname)[0];
var events = calendar.getEvents(fromDate,toDate);
var newRecurrence = CalendarApp.newRecurrence().addYearlyRule();
for (var i in events){
var ev = events[i];
var newEvent = calendar.createEventSeries(events[i].getTitle(), events[i].getStartTime(), events[i].getEndTime(), newRecurrence)
ev.deleteEvent();
}
}