I have been watching the holdover GM HD 14's of all configurations, slowly trickle away in this area. I think this Fall when the farmers and contractors determine what kind of financial year it has been, the HD's will sell much quicker. They are fairly heavily discounted as I understand, but I think those willing to spend the money on a Denali will want the latest version, even if they have to spend another 10K to get it. They will be harder to move in my opinion than the lower trim levels, strictly due to the buying habits of the people who purchase the heavily loaded pickups.
I agree the new interior of the 15's make the 14's look dated, even worse than they did already. However dated it is, it is a comfortable, and well laid out, interior I think. I am personally just tired of looking at it.