Modern publishing history abounds with "adult" writers who try their hand at books for younger readers, right across the spectrum from Salman Rushdie to Katie Price. Traffic in the other direction is far more risky and irregular.
True, some classics of the children's shelves have kept their yarns for youngsters within a cherished closed compartment of their lives. The Oxford mathematician Charles Dodgson, author of The Fifth Book of Euclid Treated Algebraically and An Elementary Treatise on Determinants,was quite happy to remain professionally apart from his alter ego Lewis Carroll. However, many giants of children's writing don't migrate readily between readers of different ages, since their fiction entirely engages heart and mind. A Tolkien or a Pullman does not write "adult fiction" because they don't straightforwardly write "children's stories" either.
If she seeks literary life after Harry, maybe Rowling should look close to her Edinburgh home. By writing an apparently juvenile romp, Treasure Island, the fragile aesthete Robert Louis Stevenson not only unlocked and unblocked his genius, but discovered an ability to exercise it over a wide range of registers, from Dr Jekyll and Mr Hyde to Kidnapped.
But the lesson from RLS is not to draw too clear a line in the sands of style and voice. If JKR does write (as rumoured) an Edinburgh mystery, a little Potter magic might speed it on its way.