This approach could really work!
First of all I am a huge fan of the series and am currently rereading them all. When I first read that this film wouldn't be a straight up adaptation, but a sequel of sorts, another iteration of Roland's quest. The subtle nod with him starting his journey with the horn of Eld is a nice touch. One that shows me that filmmakers "get" the material. This allows to mostly follow the books with taking some liberties. Think Walking Dead comic vs. Walking Dead tv show.
In regard to Roland, who cares that he is a black dude. To me it's the spirit of the character that matters, the weight and guilt he carries around with him. If it was Susannah Dean then it would be slightly different for me only because her race or at least the obstacles and struggles someone of color had to deal with especially in the time period she came from are a part of the plot itself.
Anyway, I say let's keep our fingers crossed, remain cautiously optimistic, and remember the face of our fathers.