I kinda agree with @Quryx about going from a combat unit to a non-combat unit is weird. However, at the same time I feel that a herbalist to Priest also feels weird. Thus, maybe a whole new class set would work best. Where the Priest is the base unit and then the promotions are either into a Cleric or Bishop, and the Bishop is what the New Class: Priest is, and the base class Priest does some other function. Such as helping with moral and crafting religious items.
As for what happens with the Herbalist, I made another post about adding in Research and a new Class Tier, (See Here), which could plan in nicely with the suggested Inquisitor Class.