Levels are independent from class. Rather, each class has their own experience progress. When you master a class, you gain permanent access to an Ability for that unit determined by the class you just mastered. For example, mastering the Commoner class gives you the HP +5 ability.
For the most part, you'll be running up against skill limitations when trying to reclass into a more advanced class, so sticking with a previous class to avoid wasting class exp can be prudent. But you can always reclass within the same tier, and higher tiers do have better mastering abilities.
Still, once you have 100% pass chance for a certification exam that you plan to take, take it. You can freely reclass between classes that you have passed the certification for.