Please refer to Blacksmith Pushkin in the Town of Giran to exchange a common item of A-grade for a rare item.

To exchange a Full Armor piece, you need: 4 A-grade Spheres.
To exchange an Upper Armor piece, you need: 3 A-grade Spheres.
To exchange other parts of the Armor, you need: 2 A-grade Spheres.
To exchange shield and aseccory, you need: 1 A-grade Sphere.
To exchange Low/Mid weapon, you need: 2 A-grade Spheres.
To exchange Top weapon, you need: 3 A-grade Spheres.