The effect will remain. Break Through the Line's ability cares about power only for the purposes of targeting. What happens to the creature after the ability resolves is irrelevant. And the creature will still be the same object when turned face up; it only changes some characteristics.
January 29, 2015 3:35 p.m.
Manifested creatures remain the same game object after they are flipped face up. This means that any auras, counters, and temporary effects that were applied to the face down version apply to the face up creature. Break Through the Line only has the "power 2 or less" clause on targeting so the creature will still be unblockable after it is turned face up.