If you are considering digitally changing a person's ethnicity that is a monumental task. There are other aspects involved than simply changing hue. Specular highlights interact differently on darker skin than on Caucasian skin. There are also the transitions from one skin type to another such as around the lips, eyes, palms of the hand and soles of the feet. Even the whies of the eyes are different. Another difficult aspect of skin tone are the undertones. Skin has either a pink or blue undertone. However, these undertones interact differently with differently hued melanin (the coloring agent in skin). Another difficulty with all this work is you still have to maintain skin texture integrity. Color differences aside, there are also the physical differences I'm sure you're already aware of.
I don't want to sound so discouraging but this is not an easy task you are asking to do. I would encourage you to attempt it as self imposed challenges are a great way to learn. You just need to understand the mountain you are trying to climb. Be prepared to work with a lot of making and many layers... And don't forget to post your results here.