Keep Your Shape
by Robert Sheckley
Galaxy Science Fiction (November, 1953); UK No. 13 (April 1954), pp. 4-20.
Humorous SF invasion short story with philosophical undertones. Sheckley posits a race of shapechangers whose oppressive caste system limits each individual to a prescribed set of forms. Confronted with Earth’s biological diversity, the disgruntled lower echelons exhibit a tendency to go native!