Already done, from the patch notes...

Retainers can now be assigned samurai and red mage jobs.
To assign samurai and red mage jobs to a retainer the following requirements must be met:
* Please note that in addition to the requirements listed below, players must also possess a copy of Modern Vocation.

They cannot be registered as a job. Otherwise, they must be reset.
Retainers must be a Disciples of War or Magic at level 50 or higher.
Players must be at level 50 or higher in the job they wish to assign their retainers.
I actually reset my level 60 NIN retainer as I want her to be RDM as that's going to be my primary DPS class.