I don't think they will re-cast. That's the impression I got from an interview with one or both of them because they were concerned about future films too. The films are viewed as separate entities and in terms of OotP, it is perhaps more important to establish who the "goodies" are ready for the DoM showdown. I doubt we'll see much of Grimmauld Place anyhow. The only thing of real significance there is the concept of the Order and the whole Molly Vs Sirius ordeal. Oh, plus Kreacher.