Like others have mentioned, this is not that surprising of a development. The switch in the order isn't an indication that the 787 was a bad order from NW. It just isn't the right order for the new DL/NW. The few routes where the 787-8 is needed can be served in the meantime by the 77L, and the other routes intended to be served by the 787-8 can be flown by either the 767, 77E, A332 or A333.
Not too many people realize that the -9 will have a longer range, more passengers, and a better fuel burn ratio, all at the same time, and comes only a few years later (well, they say only a few years later.) Size-wise, it is about the same as the A330-200, maybe a bit larger. And as we can see by the current shortage of A330s and especially the -200 version across the Pacific, the -9 is currently what is needed for the DL fleet. Even American ordered the 787-9, not the -8.
Maybe later the -8 will also be ordered in numbers to replace thinner 767 transatlantic routes. But not right now. If Delta gets a better deal by switching to the 77L right now and -9s down the road, then it's a good move in my books.