You should be able to edit retired schedules.
Currently the only way to edit a schedule is if it is live, which poses a risk that the schedule may run before the changes you need to make are made.
For example, I have a retired schedule where I wish to change the calendar used from a Mon - Fri to Tues only, but if I unretire this on a Friday to edit it, the schedule will run straight away, before I am able to update the calendar which would tell it not to run on a Friday.