Something that is transmutable can be transformed from one thing into another. Some characters in the Harry Potter books are transmutable, able to change from human to animal form.
Transmutable things alter in form, substance, or nature, so if your friend changes her appearance dramatically every week by dying her hair or wearing different costumes, you can describe her as transmutable. Caterpillars are transmutable too, since they transform into butterflies after emerging from a cocoon. The Latin root of transmutable is transmutare, "change from one condition to another," from trans, "across or thoroughly" and mutare, "to change."