Can plastic surgeons change a person's skin tone?

Yes.... Skin care treatments including peels and creams can do so as can laser re-surfacing and chemical peeling.
Treatments can. There are a number of treatments and products that can help improve skin tone and texture. See a plastic surgeon in your area for skin evaluation.
Changing skin tone. The short answer is no. That said, there are some bleaches that can be used to lighten. This would be fir specific treatment areas and not the whole body.