There are reports in the hadiths and the it is mentioned in the Quran where people can have a metamorphosis to a bad creature for their sins, this is called Maskh.Though is there such thing as "positive metamorphosis"? In that, a person can have a form that's different from the human form but higher in paradise?
A believer ascends and ascends spiritually such that he might become higher than the angels. He reaches a state of certainty and purity. This would be positive metamorphosis, but he would still be human.