Suppose each player will have 1 Worker.
Initially, we would assign each of them in a Unit Array (for later reference).
Unit[1] = Player 1's Worker
Unit[2] = Player 2's Worker
...
Unit[12] = Player 12's Worker
When at night, random number (between 1 ~ 12) will be rolled.
This number will determine which player is picked.
Let's say we roll and got number 5, that means Player 5 will become werewolf.
This means, use the transformation to Unit[5].
I only know a bit about Werewolf Transylvania, perhaps you could tell us more about this event ?
I have created a test map about the werewolf transformation, see if it suits your needs.