There are two methods of changing class – Promotions (via Master Seals) and Reclassing (via Second Seals). Both require a one-use item, and the character to have reached level 10 once through the playthrough.
When you change a character’s class, their level will be reset to 1 and their new weapon(s), if any, will start at level E. Any skills they had before the change will be kept and they will begin on their new stat growth.
Promotions (via Master Seals)
When a character is Level 10 or higher in their Base Class, they may use a Master Seal item to upgrade to one of two Advanced Classes, as seen in the All Classes table.
For more information about these items, see Master Seals.
Reclassing (via Second Seals)
Reclassing lets you try out various classes as well as obtain essential skills to make your units have the edge in battle. Most units have three Base Classes that they can change into. The Avatar and their children, however, can change into a Tactician and any non-special class of their gender.
To change classes, a Second Seal (see below) is required. However, the Dread Fighter and Bride classes require the DLC only items Dread Scroll and Bride’s Bouquet respectively.
For more information about these items, see Second Seals.