Ran into this last night.
Start with example line:
public static final String badlyNamed_Field = "boo";
Use the "Rename To Code Style" intention and you get:
public static final String BADLY_NAMED___FIELD = "boo";
In short, the original underscore is turned into 3 underscores. I am not sure if that is correct, but I don't think that is what I was looking for. :)
And, the final result does not cause the intention to come up again; though I am not sure if it should.