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A: Mark Rosewater, Magic head designer:
The original Phage was just a Minion. The Great Creature Type update happened during Lorwyn with a small additional update during Morningtide. The extra Morningtide update changed her to a Zombie Minion, which is what she is in current Gatherer because the Conspiracy cards are not yet in Gatherer/Oracle. [They are scheduled to go in Tuesday, June 3, 2014. —ed.]
The card was changed in Conspiracy because the current creative team felt putting Zombie was factually inaccurate and as the card had never been printed with the word "Zombie" on it they felt okay making the change from Zombie to Avatar.