I agree with Melanie here. Good design is king. If you think changing the light direction will improve the composition then go for it. Play with that idea. But keep in mind that one of the reason we use reference is because inventing realistic lighting is very difficult. In real projects, I usually don't change the light direction too much. Maybe subtle things like changing the shape of a cast shadow, or shifting a shadow a little to expose more of a cheek bone, etc. but if I decide that I want the light to switch sides, I will just search for different reference or take a different photo. Since this is an exercise targeting exploration and design, I say don't let what I just said be limiting factor. Explore anything you want.