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!