Text (UD+errata): When an opponent has 10 or less life, if ~this~ is an enchantment, it becomes a 3/2 Hound creature. [Oracle 2009/05/30]
Once it becomes a creature, it is no longer an enchantment. [D'Angelo 1999/06/01]
Once it becomes a creature, it will not change back into an enchantment if your opponent's life total goes back above 10. [Urza's Destiny FAQ 1999/05/25]
