Sometimes people can creatively use changing race and gender to good effect.

I went to see a play when I was in Chicago based on the H.P Lovecraft story The Shadow over Innsmouth (talk about your horror of racial blending motiff). The changed the view point character from male to female and it really made the themes pop in a new way.

I dont have a problem with races being changed up. It is great to see so many more characters who aren't vanilla white guy, whether they always pull it off or not. I haven't read the Dark Tower (yet) so I don't know how core Roland's whiteness is to the plot. If it is a kind of weird Sci-fi as I have heard, I would imagine not very.