Yes I agree, I would not go back into this piece because although it’s not the model’s exact likeness, it’s a totally awesome piece of art.
I would not want to mess it up. However moving forward, think two sides of the hair, light and dark. You can keep all of that wonderful watercolor in the light, while adding the model’s very dark hair value within the shadow side.
Try that next time, just change things within the shadow, not the light side.