Many shop bosses say that if you store sheng poorly, eg. open to much moisture, light, etc is can become shu. I know it's literally impossible for a raw tea to become a cooked tea, but the aging process of the tea can take on a negative aspect as well as a positive. Last weekend a boss let me try some of his sheng that had begun to take on qualities of shu and it was quite revolting.
Moral of the story, I guess, would be watch how you store your Sheng!