Transformed things are different than they once were. Think of the transformed appearance of a winged butterfly who used to be a poky caterpillar.
The robots called Transformers are a clue to what this word means — they change themselves into different shapes. Something transformed has become different from what it was: a transformed arena where people ice skate but once played basketball; a transformed hair model after a good dye job. When you see the word transformed, think "changed."