You don’t need to delete your Flex Daily schedules. Just disable them.
See this post for a pretty good way to do overseeding. Winter Rye Grass Overseeding Schedules - You can set up dates for the schedules with the start dates all the same, and taper off the end dates. Then you can just let it run without more changes. Keep the schedules around, and then just change the dates next year.
Also, since I have a rain sensor, I turn off all weather intelligence on those overseed fixed schedules. I don’t want the system to think it’s going to rain from a forecast, and then the rain doesn’t show up and it doesn’t water. I’d rather make sure that it gets the water it needs.