After your character reaches level cap, you cannot gain any additional experience unless you change your class. There are a total of four Seals which can be purchased for 1 DVP at the Staff Store in MyCastle, found inside Treasure Chests, or obtained from random villagers. They have the following functions.

Seal	How to unlock
Friendship Seal	Changes a class based on A+ support partner while retaining the same level.
Heart Seal	Changes a class based on personality while retaining the same level.
Master Seal	Upgrades a class from Level 10 onwards to a higher class.
Partner Seal	Changes a class based on S rank support partner while retaining the same level.