That's a very valid question and often it's all "in principle", as you said. InHD, for example, comes to mind. There was most likely "talk" between the companies and engineering even uplinked it for a while....
In the case of History Channel HD, this one really seems to be signed and waiting for the turn-on date.
It appears that its all signed and done. I even think that it was a History channel announcement that said that they were going to Launch on Dish Network. So with them saying that they are going to be on Dish, and Dish putting them in their advertisements, I conclude that is a done deal.
Even if a new customer was sold today it would not be installed till 2-3 days out which would be August 25th.
Even at that, there would be a small percentage of customers who would call and complain but in all my years of selling Dish nobody is going to make a stink over a week to get a new channel, where they can still watch the SD version!