It is considered lashon hora to talk about someones past negative traits. This applies even if you are trying to complement them or say in a positive way how much the person has changed for the better. Example: "Look how far she’s come. Do you know how she used to dress two years ago?"
Lesson 2
When would it be permitted to discuss negative traits? If we are doing it for a constructive purpose - specifically educating/protecting your children or students. (all the laws of speaking for a constructive purpose apply.