Actually I don't think the Demon Hunters would require the pre-patch since you are creating a new character and running it through the Demon Hunter starting zone. So that may be a possibility; however, it wouldn't make much sense to level up a demon hunter more than a few weeks before the August 30th Legion release because you'd just be sitting on your hands with nothing to do.
As far as announcing the pre-patch, I would be a bit nervous about that since they haven't tested character copy in the Beta yet and the technology behind the character copy is needed for the pre-patch. Character copy is a lot harder to implement for Legion because of all the class/spec changes, eliminating multistrike, adding the Wardrobe, etc.