I guess a more accurate way of wording this would be, "Is the change in leaves gradual, or instantaneous the moment you switch to Autumn?" My seasons are set to be 28 days long, so that might have an impact. But I'm going to be starting games in various base-game worlds to see what happens.
Edit: Okay, I think Hidden Springs is working just fine. Second day into Autumn and there are leaves on the ground. I think my green-leaved trees are turning slightly more yellowish, too. A tree that's not on the property has gone yellow-green.