The new O'Reilly Press book on the human mind (Your Brain: The Missing Manual) makes the argument that we each have a "set point" for happiness, i.e. a level of happiness which we inevitably drift back to. This is consistent with the studies showing that the increase in happiness from winning the lottery is only temporary, as is the decrease in happiness from suffering a debilitating injury.
But here's the thing: if this is true, then even though chasing happiness may be a lost cause, chasing EXPERIENCES is still just as valid.
So, maybe the lesson is not to worry so much about whether a career change will make you happy. Maybe the real question is whether the change will improve you as a person, and leave you in a better position to advance causes which matter to you.