There's apparently two ways it's not permanent.There's a lot in this game that I'm still not used to, like villager to a certain class turns out to be a perma choice from the looks of it.
The first is that the Dread Fighter class promotes to Villager. Dread Fighter is the tier 3 Merc.
The second is using the item Villager's Fork, which allows any character over level 1 to become a Villager. I don't know how you get this item or if it's expended.