Wouldn't it be better to have a wider search for the right actress, and settle on one who fits the role completely regardless of their skin colour, rather than *only* look at one particular race? Seems a little odd to limit yourself like that, but I have no problem with the change. A black Wally would be an interesting change.
Like with the Human Torch in the new Fantastic Four - while it isn't a popular decision with hardcore fans, I doubt they decided from the get-go to make the Human Torch black, rather they cast a black actor after they decided he best fit the role.