It wasn't a secret, it was fully announced it just didn't have a media blitz associated with it. For example, it was reported on in the Washington Post[1].
That agrees with the OP. Still, its not 'fully announced' if they don't have a 'media blitz' - its called a soft launch. In the end almost nobody remembered it was going to happen; almost nobody noticed.
It was semi announced that they planned on changing the recipe but at the time of the announcement that had not happened yet. It did no roll out until December and they did not announce that.
Of course from the articled submitted they said people were already saying that could taste the difference before the change was ever made.