I echo the other posters--you should definitely read the book first to find out more about SBD and how it works. Though WW Core seems to be very similar, I don't know enough about it to be sure. You can post about this in the WW forum, too, in case anyone there knows, but in the meantime, I changed the title of your thread in hopes that it would attract any other former WWers who can answer your question.
I think the best option would be to do a week of Phase 1 just to clear away anything that might be in your system. Don't expect any major weightloss, since you've been doing Core, but expect it to ensure that you're free from cravings.
Hope that helps!