Here's the thing: Uhura is not REPLACING McCoy on the poster, because, as mentioned before, McCoy was never featured all that much on the posters to begin with. All talk of the "Big Three" aside, that's never been reflected on the movie posters. That's been standard operating procedure for over thirty years, not "proof" of some dire new trend.
different is Uhura is being featured more prominently. But she's not replacing McCoy on the posters. She's replacing Ilia, Saavik, etc.