An amulet of unchanging prevents you from changing your current form by polymorph or other means, and also protects you from being slimed; any sliming in progress is aborted. It does not prevent you turning to stone. Another exception is prayer as a lawful in undead form: you will rehumanize anyway. When eaten, you are changed back to your natural form. If you die while polymorphed and wearing an amulet of unchanging, you will not revert to your previous form, and will instead die.