How would it take some time for news to notice? In the wake of this latest crash, news media and amatuers alike are constantly paying attention to what airlines with the 737 MAX-8 are doing. If they notice that airlines are starting to reschedule their flight plan to use other aircraft than the 737 MAX-8, the news will be out instantaneously.

Indeed, if they do it in secret (attempt to anyway), it will probably be a lot worse than a public statement. I just don't see how you can hide that major a change (I think flight plans are public anyway) without someone noticing immediately in this day and age.