Rulings
A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
(wotc • 2008-08-01)
Although Mutavault has all creature types while it's animated, it does not have the changeling keyword ability.
(wotc • 2008-04-01)
Rulings
A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
(wotc • 2008-08-01)
Although Mutavault has all creature types while it's animated, it does not have the changeling keyword ability.
(wotc • 2008-04-01)