That's not to say requirements can't change - of course they can. But they shouldn't change just because I forgot to make myself clear the first time.
Either communicating the course, or if upper management fails in their duties, charting the course and communicating, is essential.
For example, "we will deprecate this technology by XX date, and we have cleared the path to enable this" is often essential to success. In a healthy org, telling people what you're doing, how you're doing it, and why you're doing it is as necessary as when you're doing it.
Just like you don't want to release every feature all at once, you similarly don't want to muck up your infrastructure (physical or business).