Fire Emblem: Awakening contains highly customizable unit upgrading and jumping, which are achieved through Master Seals and Second Seals, respectively.
Once a unit reaches level 10, he or she can use a Master Seal to upgrade to the next class. Some units can be promoted to one of two different classes—for example, the Myrmidon can become either an Assassin or a Sword Master. Luckily, you can always compare your prospective stat changes and what skills you’ll learn on the bottom screen.
Second Seals, though, allow your unit to change class once they’ve reached at least level 10 (or, if they’ve achieved Advanced Class). The new class will start at level 1. Any unit can use this, but which class they can change to is limited. For example, the Cavalries can hop into Archer or Myrmidon classes. Even the Lord can hop classes, becoming a Cavalier or an Archer. In addition, you can use the Second Seal on your Advanced Class unit to “promote” them to their previous class at level 1.
(On the other hand, your Tactician can change into literally any class, from Cavalier to Barbarian to Wyvern Rider. Oh, the options.)
It looks as though there are two main purposes for this other than simple unit preferences. The first is that because each unit learns two skills as they level up, this will allow you to collect more skills. For example, turning your unit into a Thief will automatically grant him or her the Locksmith skill, allowing them to open any door or chest without a key. Then, if you decide you’d like your original unit back, you can transform the unit back and still keep the Locksmith skill.
The other purpose is to allow easier maximization of stats. Because you start at level 1 every time you switch, you will always have the opportunity to level up. Hit the maximum level of 20? Just use a Seal and jump class! Maximized your Advanced Class unit? Use the Seal to jump back to level 1 of your current class while keeping your stats intact.
Something to be noted, though, is that every unit does have their own natural stats, so some classes may just not suit them, and class-specific bonuses will be lost between jumps, so sometimes there will be stat decreases after using the Seal.
Here’s more of our Fire Emblem: Awakening coverage in case you missed it: