Shapeshifter | Magic: The Gathering
Formats Shapeshifter is Legal in
Official Oracle Text for Shapeshifter
At the beginning of your upkeep, you may choose a number between 0 and 7.
Shapeshifter's power is equal to the last chosen number and its toughness is equal to 7 minus that number.
Official Rulings for Shapeshifter
8/1/2008 : The ability that lets you choose a new number is a triggered ability that triggers only once each upkeep.