Is aggression something that can be fixed or cured? This is something that I have a lot of heart to heart talks with students over, and the short answer is...: no. At least not in the way many people think of it. However, this situation isn't as helpless as that answer might lead you to believe. While I would never consider aggressive behavior "fixed" or "cured", I will say that dog who has previously exhibited aggressive behavior can be managed properly and their behavior can be modified so that the chance of falling back on that aggressive behavior becomes minute.
So, why won't I say aggression is "fixed"? Because those neural pathways will always be there and will always have had some reinforcement history (in this case we're discussing negative reinforcement, in that the growling/snapping/biting made a bad thing go away, and that will increase that behavior in the future), and thus if the dog is presented with the same trigger and put in over his head he may fall back on that old behavior. With many dogs who may be predisposed to worrying, half of the battle is managing the dog's environment and setting him up to succeed, teaching him new skills to cope with whatever he was aggressing at before.
So what about dogs that trainers claim they have "fixed"? I will say this, most of the trainers making these claims are heavily based in punitive training and while they see a "fixed" or "calm-submissive" dog, as one famous TV personality might put it, I see a severely stressed dog who may have achieved learned helplessness after some pretty awful flooding. This is a dog who has given up all hope of escaping it's trigger, whether through fight or flight, and has emotionally shut itself down to protect themselves. This isn't "fixed", this is even more broken than before.
So should you despair that you can't "fix" your dog? Absolutely not. You should rejoice in the fact that you have the opportunity to teach your dog new skills to cope with what previously would have sent him over threshold, and a way to deepen your relationship and your dog's trust in you.