Create a new rule called Classes:

Each Adventurer has a Class tracked in the GNDT under a column called “Class”.  Classes may be Basic or Advanced. A Basic Class is one item of the set {Wizard, Fighter, Rogue, Monk, Sorcerer, Cleric, Barbarian, Bard, Paladin, Ranger, Peasant}.  All non-Basic Classes are Advanced Classes.

New Adventurers start with a Class of “Peasant”.  An adventurer with a Class of “Peasant” may set their Class to any other valid Basic Class at any time.

Set each Adventurer’s Class to Peasant.