Honestly it depends on the origins of the aggressiveness and how adaptable the dog is. Not all aggression comes from genetics. Alot of it comes from improper training or a serious lack of training/socialization. People don't realize that while dogs have been bred to be companions they aren't all equal and that with some breeds there's extra care needed when it comes to putting them in environments that they weren't originally intended for. A lot of people approach dogs as if they're prepackaged with good behavior, not realizing the amount of work it takes to teach an animal to behave, let alone communicate with it.

Once they hit a certain life stage it's not easy to overcome the lack of good training and socialization that would have sufficed at an earlier moment in their life. Some experts recommend that puppies be socialized with something like 40-50 dogs of differing sizes and ages and meet something like 200 people, from small children to older people of all walks of life. Obviously this will take time but dogs will greatly benefit from this exposure because of the way their brains work. Strange things, especially as adults will be treated as something unsafe or possibly a threat. So getting the dog greater exposure to all sorts of people, things and dogs greatly reduce the chances of aggression.

The one type of aggression that is almost impossible to deal with is the kind that comes from temperament. This is something passed on from the dogs parents and is so hard-coded in the dog that most dogs experiencing this often end up put down over it. You may hear of the occasional feel good story of someone working with one that somehow or supposedly gets better. Regardless of how true that might be in that particular case. Most of these types of dogs don't live long happy lives.