According to the Who Do You Think You Are website JK Rowling will be appearing on the first episode of the season, the date has not been set.  The description of the episode is as follows:

Best for…French connections

Across the course of seven books and eight films, Jo Rowling’s Harry Potter series has captured the imaginations of millions of people around the world. Who Do You Think You Are? sets out to explore the French ancestry of the author’s mother.

Thanks Snitch Seeker