I think that whether or not you're going to write something using "old canon" after new comes out depends... I can understand coming up with a long, involved plot or a specific idea to explore that gets jossed before you get a chance to write it. But to specifically brainstorm things based on old canon seems weird to me, as if it's picking and choosing the elements of a universe to which you'll comply. Which, yeah, we can do, but that seems to take some of the challenge out of writing fic.

Of course, I tend to write in the far-flung cracks and crevices of the Potterverse that will likely never be confirmed nor denied by canon, so what do I know.