Text (SM): At the beginning of your upkeep, you may exchange control of target nonland permanent you control and target nonland permanent an opponent controls with an equal or lesser converted mana cost.
You can't play the ability unless you can find two targets that meet the requirements. [Shadowmoor FAQ 2008/04/21]
You must choose targets is a legal set is available. On resoltion of the ability you decide whether or not to exchange. [Shadowmoor FAQ 2008/04/21]
If either target becomes illegal before resolution, no exchange is made. [Shadowmoor FAQ 2008/04/21]
Remember that tokens have a converted mana cost of zero unless otherwise specified by the effect that created the token. [D'Angelo 2008/07/12]
