Don't know if it's of help but I can tell you how I use prep and my experience. Often for October when I take part in the challenge, I just prep more than one novel since I don't always know what I am gong to write. There have been at least one year, if not more, where I didn't decide on what to write until the week or even minutes before November started.

On the plus side, even if you don't write the novel you prep this year, you will have all of the prep work done for the future. That's why I do 2-4 novels when I prep. Whatever I don't use in the near future, they still are things I would like to write some day and the information will come in handy later.

This year I'm prepping 4 novels. I had an epiphany and decided what to write for NaNo a couple of days ago. It's not a novel I'm prepping. Sometimes that is just how it goes. *shrugs*