Once we have two 5 stars same grade same artist (not necessarily have to be the same theme/album), we can "upgrade" this card to a higher grade.
For example: I upgraded 2 5stars "Red Velvet Irene" C cards for one 1star "Red Velvet Irene" B card. It costs 5000R to upgrade C to B card.
Good news: 100% chance of successful upgrade.
