Probably you are right, the cost/quality ratio of trying to replace smoothly the boy by a girl might not balance just redrawing everything from scratch, since no fine image editing is required in the latter case.
As for the copyright, the raw files provided by the author himself are available for free
here and
here (you just have to register on the website to download them), so I guess there would be nothing wrong with editing and distributing it also for free.