The Lawbreaker powers have always kind of annoyed me, so I can see why you'd toy with changing them. I understand why they were put in (ie, falling to the temptation of DF's equivalent of The Dark Side), but even so, I disliked the idea that it was either avoid breaking the Law like the plague or immediately lose your character (because most spellflingers are likely not to have refresh to spare, as mentioned by others.
I like your idea for toning it down a bit, and I think the key to it working will be how you implement the aspect changes. The weak link might end up being the player who doesn't really care about playing up the whole Lawbreaker drama thing, so to speak, and just designates one of his aspects as being his "Lawbreaker slot". Then any time he's told he has to , he just makes a minor tweak to it. "I broke the 1st Law" -> "I broke the 1st Law many times" -> "I broke the 1st Law many times, and the 2nd Law, too!" Which is pretty boring, in my opinion.
Instead, you might want to rule that the player has to taint a different aspect each time its called for, and make the change a team effort to guarantee that the change is interesting and also reflects the darkening of the character's soul.
As an aside, the "benefits" of the Lawbreaker power can even be represented through the aspect change, without needing the power. Just let the player invoke the tainted aspect when breaking the aspect again (which probably doesn't even need to be counted as a house rule). Or even allow them to tag the aspect for debt (though its probably worth limiting how often this can occur -- maybe once per scene, or up to a limit of X debt).