One Sheep on an Island of Wolves

There is a pack of exactly 99 wolves on an island living alongside exactly 1 sheep. Usually, the island herder protects the sheep from the wolves, but he's leaving for a few days. Before he leaves, he casts the following spell on all the wolves, in an effort to protect the sheep:

If any wolf kills and eats the sheep, that wolf itself will turn into a sheep.

Each wolf can eat the grass on the island, but they much prefer the sheep to the grass. Assume each wolf is perfectly rational. That is, they definitely don't want to turn into a sheep and then get eaten by another wolf, but they would prefer to eat the sheep (and live forever as a sheep if it knew somehow that it wouldn't get eaten) to eating the grass. Only one wolf can eat one sheep.

The herder leaves the island and expects all 99 wolves and 1 sheep to be remaining when he's back. Has he made a mistake? Does the sheep get eaten?

Source: The green book