Once it becomes a creature, it will not change back into an enchantment if your opponent's life total goes back above 10.

Once it becomes a creature, it is no longer an enchantment.

A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of his or her most recent turn. It doesn't matter how long the permanent has been a creature.