Personnel Management Data Hack

I don't have much to say about the recent hack of the US Office of Personnel Management, which has been attributed to China (and seems to be getting worse all the time). We know that government networks aren't any more secure than corporate networks, and might even be less secure.
I agree with Ben Wittes here (although not the imaginary double standard he talks about in the rest of the essay):

For the record, I have no problem with the Chinese going after this kind of data. Espionage is a rough business and the Chinese owe as little to the privacy rights of our citizens as our intelligence services do to the employees of the Chinese government. It's our government's job to protect this material, knowing it could be used to compromise, threaten, or injure its people­ -- not the job of the People's Liberation Army to forebear collection of material that may have real utility.

Former NSA Director Michael Hayden says much the same thing:

If Hayden had had the ability to get the equivalent Chinese records when running CIA or NSA, he says, "I would not have thought twice. I would not have asked permission. I'd have launched the star fleet. And we'd have brought those suckers home at the speed of light." The episode, he says, "is not shame on China. This is shame on us for not protecting that kind of information." The episode is "a tremendously big deal, and my deepest emotion is embarrassment."

My question is this: Has anyone thought about the possibility of the attackers manipulating data in the database? What are the potential attacks that could stem from adding, deleting, and changing data? I don't think they can add a person with a security clearance, but I'd like someone who knows more than I do to understand the risks.