I don't think most people would consider IQ a 'character trait'... However, that's a matter of terminology and doesn't negate your point. I agree that 'fluid intelligence' is probably relatively innate and would be hard to change (although there's some research that training tasks such as the dual n-back can have an effect.) Crystallized intelligence, as basically the sum of your knowledge and ability to apply it, can definitely be increased by practice. IQ in isolation strikes me as something that wouldn't matter as much as IQ and amount of experience and good work habits and openness to criticism and improvement.

As for conscientiousness, I have no idea what kind of research has been done on its stability as a character trait, but I see no reason why someone who was aware enough to make a decision to become more conscientious wouldn't be able to train themselves in habits that would, at the very least, make them able to get more work done and appear harder-working to others.