Well she can just choose to wear the same clothes and have the sane haircut she had as a kid. I don't see why it's necessary to justify it.
OK. I hope I haven't made trouble for you. I still really like your artstyle and would have played the updates to the game regardless of if you changed the beginning. I asked more out of confusion as to why the game started like that rather then out of not liking the demo. Its not like anything happens to Gwen while she is underaged.
Still if you think changing the demo will allow you to make the best possible version of the game, I say go for it.