Of course, one of the things you can do with a minor milestone is change an Aspect. Lawbreaker makes you change an Aspect to reflect your lawbreaking; it says nothing about the changed Aspect being more permanent and unchangeable than any other Aspect. Unless you're breaking a Law multiple times every session, it's possible to stay ahead of the game and take the lawbreaker references back out.
Keep in mind that while a milestone does allow the option of renaming an aspect, the rules also note that "Obviously, these changes should be justified as much as possible, either within the story ... or as a result of play". Given that both the story and play were what justified the shifting aspect to begin with, I don't see how it would be possible to justify changing it back at the next milestone. Not to mention that the altered aspect doesn't represent some afliction for which you might look for a cure, but rather an actual change to who you are.
That said, I
could see it changing back with time. For example, a budding warlock-in-training gets started on the wrong path, breaks a Law a couple of times, and sees nothing wrong with it. Then something happens to make it hit home. Perhaps he badly hurts someone he cares deeply for, and suddenly the horror of what he's becoming becomes clear to him. He resolves to make a change, and ceases using his magic in ways that are even close to the line, and perhaps finds other ways to atone. After a time, he internalizes these practices, turning them into habits; in effect making them part of who he is. That's when the aspect would be renamed, and it probably out to be renamed in such a way that it brings out his struggle and ultimate success.
For example, imagine an overly-zealous Warden who decided to fight fire with fire, as it were. He starts out with the aspect "I am the Law!" In an effort to more effectively eliminate the threat of Warlocks, he begins using his magic more aggressively than he should, and soon finds his aspect changed to "I am the Executioner!" One day, after a harrowing fight with a nasty Thing That Goes Bump, he returns home, and senses a spell building there. He goes in with his magical guns blazing, as it were ... and finds that he has just killed his young apprentice, who was innocently practicing a spell. How he avoids a run-in with the other Wardens is a story of its own, but lets say he does, and manages over time to atone for his crime. He carefully avoids using lethal magic, even against the nastiest of Warlocks he faces, because he's realized the danger of following that road. Finally, his aspect changes one last (we hope) time, this time to "The Law is what seperates us from the monsters we fight!"
In general, I'd treat the Lawbreaker aspect kind of like an extreme consequence (because in a way, that is what it is).