Aura swap

Aura swap is a keyword ability. It lets you exchange an Aura that has it for an Aura in your hand.

Description
Aura Swap was first featured in Future Sight. By nature, this is a blue ability and it has only been printed on one Aura so far, Arcanum Wings. Mark Rosewater considers the chance of it ever being reprinted likely, probably in a set with an Aura sub-theme. But he also consders it a dangerous mechanic, in that it lets you circumvent mana costs.

Rulings

 * As the aura swap ability resolves, you may choose an Aura card in your hand. If you decide not to choose one (or don't have one), nothing happens. If you do choose one, the game checks whether that Aura can enchant the enchanted permanent as it currently exists (while the Aura with aura swap is still on the battlefield). If it can't, nothing happens. If it can, and the Aura with aura swap is still on the battlefield, and you own the Aura with aura swap, then the Aura with aura swap is put into your hand and the chosen Aura is put onto the battlefield attached to that permanent.
 * The exchange is simultaneous. At no time will the permanent be enchanted by both Auras or by neither Aura.
 * If a non-Aura somehow gains aura swap, the ability will do nothing when it resolves. The Aura in your hand can't be put onto the battlefield enchanting nothing, so the exchange doesn't happen.