The " shape shifter " concept has a lot of appeal to people who are fond of conspiracies. The notion that a physical body can alter it's shape and appearance allows for the premise of the aliens to infiltrate into any branch of society that they wish. Lets examine this closer. In order for a physical body to change mass , either in becoming larger or smaller, that mass must either be accounted for by taking in mass to grow or shedding mass in order to become smaller.Therefore in order to become larger, a large amount of nutritional material would have to be consumed and assimilated. In order to become smaller, the mass would have to be excreted. The process itself would require a certain amount of energy. The greater the change, the more energy would be required.The greater amount of energy required is going to produce heat as a waste byproduct. This process is going to place severe limits on your shape shifter theory. Polymorphism is just too demanding of physical laws of nature. Aliens that resemble us I can beleive, mind control of existing humans is beleivable. But polymorphism is just too big a stretch of the imagination.