If the GM has set a minimum length of time for characters to change their power facts (such as "no more than once per session"), characters with the ability to mimic others' powers, or to radically change their own, might require a stunt that lets them change power facts more rapidly.
Characters who can adopt a variety of common powers (such as shapeshifting into animals) usually shouldn't need such a stunt. Instead, their power facts would include things like "Able to adopt the physical traits of animals".