Need to edit a repeating event in Calendar, but it won't let me as it's greyed out. I don't want to have to delete it and make an all new event, because I want the history to stay on my calendars. Is this a bug or am I missing something?
Not sure. Is it an invitation event? Those are different by design.
For deleting non-invitation repeating events, Calendar usually asks me if I want to "Delete All Future Events" or "Delete Only This Event", leaving events in the past alone.