I have thought about that scenario from time to time.
First thought is to look to other evidentiary issues such as dinosaurs. Although the Watchtower has never commented on dinosaurs, to my knowledge, growing up I have heard Witnesses say that dinosaur bones were placed in the ground by demons to get mankind to believe there is no god. I have also heard Witnesses claimed that UFO sightings and extraterrestrial encounters are also demons doing the same thing.
I think the question should go beyond just Jehovah's Witnesses. What would most of Christianity do if undisputable evidence of intelligent extraterrestrial life were found? So many evangelicals will look you right in the eye and tell you that the Earth is only thousands of years old.