it's actually not a good idea to do time expired metamorphosis into flying units, because if you are over water or over trees when the metamorphosis is over then you run into BAD problems.
this happened to me when i was creating the same sort of ability.
if they meta back over water, they die.
if they meta back over woods their elevation doesn't change properly. i would avoid making this ability last a certain amount of time, but instead, a permanent amount of time until they want to switch back to the dreadlord... like in Dracula's Curse.
you could base this type of ability off of the druid of the talon's storm crow ability.