I'm pretty sure it'll work based off your retainer's current level. When I changed my Gladiator to a Paladin she kept the same level. All changing the retainer's class to a job does is allow access to the job-specific gear, that's all. Unlike us players, they can function perfectly well without a job. The reason for this is retainer's progress and what they can get from ventures is determined by their level + item level, not the abilities required.