Once a unit reaches LV5, they’re eligible to take Certifications for Beginner Classes. However, take note the unit’s skill level will affect his/her odds of passing. If they reached the minimum rank for the required skill(s), the odds of passing is automatically 100%. You can still gamble and take the Certification even if the unit's odds of passing is below 100%. However, if the odds of passing are below 30% (usually happens if the unit's skill's rank is below the required minimum), a unit can’t take the certification for that particular class.
In this page we will cover all the Beginner Classes in the game, including their required skill levels. Before switching to a new class, you can also check the Stat changes by pressing ZL/ZR. Red values are Stats that lowered upon Class Change while blue values are stats that are boosted. Press X to display the amount of stat points to be deducted or boosted when switching classes.