It depends. I know, this isn't the answer you were hoping for but this is a tough question.

If the nail changes are due to fungus, treatment can often improve the apperance of the nail, but the severity of damage from fungus can limit HOW improved the appearance gets.

If the nail changes are due to trauma, there are some topical agents that may improve the condition of the nail.  Again, it depends on what we are starting with and how the treatment protocol is followed.