Transform

Transform is a keyword action introduced in Innistrad and means to turn a double-faced card on the back side, changing the card face that is visible.

Description
Each double-faced card has a front face. This face is the default; double-faced permanents start on the battlefield with this face upwards. If an ability instructs you to transform the permanent, you turn it over so its other face is up. Whichever face is up at the time tells you what that permanent is and what it can do.

History
Dark Ascension introduced some different transforming cards; the first transforming artifacts, the first creature that returned from death transformed, and the first creature that transforms into an aura. Magic Origins has five legendary creatures who are exiled and then returned transformed (into planeswalkers) from the exile zone. Shadows over Innistrad and Eldritch Moon saw a sorcery that transforms from the graveyard into a creature, an Aura that returns from the graveyard that transforms into a creature, and an artifact that transforms upon death from a creature to an equipment. Ixalan block had all artifacts or enchantments that transformed into lands. Core Set 2019 has another creature-into-planeswalker flip card and follows the Magic Origins format.

Rulings

 * "Double-faced" always refers to the physical card. It is neither a characteristic nor a copiable value. For example, a token can never be a double-faced permanent even if it's a copy of one.
 * Each face of a double-faced card has its own name, types, subtypes, power and toughness, abilities, and so on. While a double-faced card is not on the battlefield, consider only the characteristics of its front face. While a double-faced card is on the battlefield, consider only the characteristics of the face that's currently up. The other set of characteristics is ignored except for the converted mana cost. The back face has by definition the same converted mana cost as the front face.
 * A transforming double-faced card enters the battlefield with the front face up unless it is stated that the card enters the battlefield transformed.
 * The back face of a transforming double-faced card can't be cast.
 * Transforming a permanent doesn't affect any Auras or Equipment attached to that permanent. Similarly, any counters on the permanent will remain on that permanent after it transforms.
 * Damage marked on a double-faced permanent will stay marked on that permanent after it transforms.