Re-assigning Tasks

We have a cook and an assistant cook in the same center we would like to have change positions. The cook is just not getting a very key part of her position involving record keeping (this despite much training). She is very good at other parts of her job. The assistant shows a stong aptitude for the record keeping piece, which is part of the cook - but not assistant - job description. What would be the best way to reassign duties without getting into adverse employment stuff?
Comments
I would not worry about any adverse personnel actions for this is based on well documented failed performace events.
PORK