If you exile a card to Werewhat’s enters ability that is already a double-faced card, you may choose either face of the card to serve as the back face of Werewhat. The face you didn’t choose would be treated as nonexistent for as long as Werewhat remains on the battlefield. If your choice somehow ends up with an instant or sorcery card as the back face of Werewhat, Werewhat would be unable to transform away from its daybound side, so it would remain daybound even if it’s night.