Enchantment, 3UU (5) Whenever a player chooses one or more targets, each player reveals the top card of his or her library. The player who reveals the card with the highest converted mana cost may change the target or targets. If two or more cards are tied for highest cost, the target or targets remain unchanged. Changing targets this way doesn't trigger this ability.1
Illus. Ray Lago
You can change all, some, or none of the targets. [Invasion FAQ 2000/10/03]
You cannot change the number of targets, choose an illegal target, or make something get targeted more than once. [Invasion FAQ 2000/10/03]
An empty library will have no card to reveal, so a player with an empty library has no card to compare. The effect looks for the highest cost card of the cards that actually are revealed. [D'Angelo 2000/11/17]
With the Split Cards (see Rule 505), use the higher of the two costs on the card to compare with the other card. [DeLaney 2000/10/30]
Invasion
(Rare)