I think it is totally ok to change your mind partway through. I certainly have, in the past. And Prep is a really good time to find out that the idea you had swimming in your brain just isn't clicking this time.

If you truly don't like it, I'd say to change it. Don't keep fighting with something that isn't working. I tend to lay down, eyes closed, with a warm or cold pack on my neck (depending if I have a headache or if I want to relax), make the room quiet, and just... dream/think. Some great stories can pop into a brain that way.