Take note that special Classes doesn't have advanced classes so their level cap is LV40 and they can learn 4 skills along the way, similar to how base units promoted to advanced classes to learn their skills. Carefully consider which character you want to change to these special classes since the "seals" or catalysts required for the class change is consumable and very rare. Free DLC catalysts are rare but if you purchased the DLC where the catalysts will come from, you can get them but you still have to complete the DLC mission to get them as reward.
More information about the DLCs are found in the game's official DLC page.
Sprites courtesy of Fire Emblem Wikia