You unlock upgrades at 5,000 exp.
You have to have X of the same unit to upgrade it to the next tier up, as well as Y dragon coins
- Common: X = 5, Y = [25,100,500]
- Rare: X = 4, Y = [75,300,1500]
- Epic: X = 3, Y = [150,600,3000]
- Legendary: X = 2, Y = [250,1000,5000]
where the first value of Y is for the Basic to (+) upgrade, the second is for (+) to (++), and the third is (++) to (+++).
For example, combining 5 Pawn+ units into a Pawn++ requires 100 dragon coins.
Upgrading a unit typically raises the unit's army point cost and value equivalent to morale. It also buffs the unit.