The storyline and scripting for Final Fantasy VIII was finished by the time the demo was released to consumers and stores. In the development process, the storyline and scenario development are typically finished before the actual development of the programming and engine. This is why there is usually content that you can find on the disc that was never used during the game. I have no doubt that Selphie was already a character with a storyline by the release of the demo and that Rinoa was used in her place for marketing purposes and for the sake of difference between the demo and the final product.