Hm, I feel like this is way less sinister than it sounds. From what I know of his prior reputation, Will Happer has long made his position on climate change clear, so it's not like he changed his mind when people offered him a big pile of money. If you already believe something and someone is going to give you money to promote your views, why on earth would you say no?
We cannot know what his original view on climate change might have been. Even if he was on the conservative side to begin with, there's a large spectrum from "Earth might be getting warmer, but I don't think it's anthropogenic" to "CO2 is actually good for us!"
Perhaps some of his previous papers were paid for by others with similar interests as well. Judging from the way he tried expertly to hide the source of funding and avoid proper peer review, it certainly doesn't sound like this is the first time he's done it.
Moreover, humans are remarkably good at changing their beliefs in the face of cognitive dissonance. Someone who is repeatedly paid to advocate a certain point of view might honestly come to believe that the bullshit he's been spewing is true, and gradually move to a part of the spectrum that is much more radical than where he started out.