Some units can evolve. If this is the case for your unit, that unit will evolve into a better job class, and never a worse job class.
Once a unit has evolved, it can't devolve, i.e. go back to its earlier class.
Here's an example: Say you have a Dragoon unit. This Dragoon may be capable of evolving into an Arc Dragoon. Overall, the Arc Dragoon class's abilities are relatively better than those of the Dragoon class.
Because units never devolve, an Arc Dragoon can never revert back to a Dragoon.
The number of times a given unit can evolve is determined by the number of units you have at that given moment, so keep this in mind as you strategize!
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