User blog comment:DaNASCAT/Technical Update: August 22, 2016/@comment-25356303-20160823154448

We don't seem to be able to edit Discussions post categories. Users seem to be occasionally putting their posts in the wrong categories, so it would be nice if we could fix it for them.

I might just be missing something, though. Haven't had the chance to try it on the desktop version, stuck on mobile for now.