Switching Eddie would impact the story way more. Roland being from the line of Eld means absolutely nothing as the line of Eld is completely fictitious. Other than the "looking like King" thing there is really nothing in the story that would be impacted by Roland being black. That is untrue with both Eddie and Susannah. Honestly they should get the best actor they can. If that happens to be a black guy and it doesn't require changing the story to do so then I say go for it.
Them giving legs to Susannah would be a much greater crime.