Class Evolution, also known as Class Change or CC by players of the Japanese version, is the process of enabling a Unit to unlock its full potential. A Unit that has been CC'd will be reset to level 1 but with higher stats than the level they were before being CC'd. From then on, depending on the rarity, the Unit will be able to gain an additional 50 or more levels in addition to whatever bonuses they received upon Class Evolution. Only Units of Silver rarity or above are capable of Class Evolution, however, there are exceptions that cannot be evolved this way, like Princesses and certain event Units.
In order for a Unit to undergo Class Evolution, they must be at a minimum of level 30. However, it should be warned that a Unit that undergoes Class Evolution before they reach their maximum level will never be able to regain the stats they missed out by CC'ing early. Due to this, the difference in stats between a Unit that was CC'd at level 50 and a Unit that was CC'd at level 30 is very large. The sole exception to this is the Unit Sealed Dorania, whose maximum level is 30. Despite this, there will be some cases where it is better to CC a Unit at level 30 in stead of the maximum level. Examples include Fodder for Skill Level Up and Awakening.
Once a Unit has reached level 30 or higher, it is possible to Evolve them first by going to the "Unit" menu, and then selecting the "Class Evolution" option. In this screen, all Units under level 30 are grayed out and unselectable. In addition, any units that have already Class Evolved or are incapable of Class Evolution, like Princesses, will also be grayed out.
By selecting one of the non-grayed out Units, their stats will be displayed to the left along with the option of "Select". Clicking this will bring up a screen that lists the materials required to Evolve the Unit. In general, this will consist of 3 Class-specific Units and one Spirit of the same rarity as the Unit you are trying to CC. However, there are occasional exceptions to this rule, usually in the form of Event Units, like the Unit Sealed Dorania.
Depending on the rarity of the Unit that will be Evolved, the rarity of the materials will also Change. For Silver and Gold Units, this will consist of 2 Iron Units, 1 Silver Unit, and 1 Spirit of Silver or Gold. For Platinum and Black Units, this will consist of 3 Silver Units and 1 Spirit of Platinum or Black. For the currently unique exception of Sealed Dorania, the required materials are 1 Bronze Dragon Soldier Unit, 1 Silver Dragon Soldier Unit, and 1 Spirit of Time, all 3 of which are only available during the Return of the Dragon Princess Event.
After obtaining the necessary materials for the specific Unit's Class Evolution, pressing the "Evolve" option will initiate a small cutscene where the Goddess Aigis empowers the unit and changes their battle sprite. Once the cutscene is over, the Unit's level will be reset to level 1, and it will instead gain a small amount of stats in comparison to what they were before the Class Evolution.
For each CC'd Unit, they will receive a new maximum level that they can reach. With the exception of Silver Units, who remain with a maximum level of 50, Gold Units can reach level 60, Platinum Units can reach level 70, and Black Units can reach level 80.
In addition to a new maximum level, many Units will also receive and upgraded version of their skill. However, this is not always the case and generally depends on the specific Unit. In the case that the Skill changes, whatever level the previous skill was before Class Evolution will not be kept. If the Skill stays the same even after Class Evolution, then the Skill Level will also be kept. More information can be found in the Skill section of this wikia or on a specific Unit's page.
Other additions that are gained from Class Evolution are various changes in abilities depending on the Class, a change in Class Name (ex. Soldier to Soldier Chief), and an increase in Unit Cost. For Units that have already been Cost Reduced prior to Class Evolution, they will keep a reduced cost depending on how many times they have been reduced. More information can be found in the Unit Combination section of this wikia.