HBO has officially greenlit a second season of its upcoming “ Harry Potter ” series, which will debut its first season this Christmas. The news aligns with the network’s plan to adapt all seven of J.K. Rowling’s “Harry Potter” books into seven seasons of television over the course of a decade. Season 2, which will be based on the book “Harry Potter and the Chamber of Secrets,” will begin filming this fall. Jon Brown, a writer on the first season of the adaptation, has been promoted to serve as co-showrunner on Season 2. He joins current showrunner Francesca Gardiner, who said in a statement, “As we have laid out our plans for the overlapping production schedules to finish Season 1 by Christmas and to return to production for Season 2 this autumn, it has become clear that bringing on a co-showrunner is the key to maintaining our momentum.…