Well, if it's a permanent effect it won't have a maintenance cost in mana. It also would make your soldiers werewolves which would be a generally bad idea in any army you were trying to control. But as we're making this all up we can define the lore however we like, lol.
The thing is, how much werewolf lore do we stick with? Why would anyone turn one of their units into a group of mindless killers that are going to eat whatever peasant they can get their teeth on every full moon.
How does a unit of werewolves act? Do they obey orders? Do they kill their allies? Do we track the moon?
My thought on this was that the spell allowed the members of the effected unit to take "hybrid form" at will. They get all the benefits of being a werewolf, none of the detriments and there would be a maintenance cost in mana each turn.
If we want more "realistic" (that makes me smile) werewolves how should we go about it? Should it change/remove this spell?