Upgrade Monsters are type of monsters that is part of your Extra Deck. Their based color are pink.
Originally, Upgrade Monsters are like Ritual Monsters. The similarity is that they need to be summoned by spell cards. However, the difference is that Ritual Monsters can be summoned by tributing a monster while Upgrade Monsters can be summoned when a monster is on the field as their current form.
Unlike Xyz Monster with their Rank, Upgrade Monsters have Grades. These grades must be the same amount as their former form's level.
Plus when they're destroyed, Upgrade Monsters are sent back to the Extra Deck.