Diversity promotion in Hollywood is not new, and to me, it's not a huge deal so long as the studio isn't doing it to say "look at us, we're so diverse". That's where diversity promotion becomes cringe-inducing.
That being said, there are sadly some unavoidable issue when you change the race of an established character. As mentioned by some above, those who are already well versed in the series will be disenfranchised with what they see as an unnecessary change to a beloved character. But there's another issue that's a bit more practical from a marketing standpoint imo.. for someone who isn't familiar with the series, when they see a non-white character in the show and then go play the games or read a book with that same character in it, they'll get pretty confused and question if they're mistaking one character for another.
While the altruistic goals of diversity are noble, they get bogged down in these circular discussions because we can't go back in time and re-diversify a previously established series. Thankfully though with Halo, there are a bunch of nationalities and cultures represented throughout the series. So for me, I guess I'm just confused by the decision to inject diversity in one place when there are already other places within the series that provide that opportunity. It comes off as a tad lazy, but to be fair, it's awfully early for me to definitively say that the casting choices are a mistake.