A Uuvo is the Ultimate Evolution of a card’s existing Ultimate Evolution. Like other Ultimate Evolutions, the card does not need to be Max Level before evolving, and some Uuvos even keep the cards skill level intact (though not all; always check to see if the active skill names match before evolving to be sure!).
Like standard Ultimate Evolutions, Ultimate Ult. Evolutions can be reversed by using one of each Lit as an evolution material and selecting the Ult. Devolution option when in the Ultimate Evolution Select screen; as shown here with Ceres Armor, Ana :