Poltergeist: The Legacy (1996) : 4x19

Kat faces mortal danger when she resumes her friendship with an ailing Miranda. To save Kat, Rachel must find powers from the past, a past she would prefer to forget. For this episode, we learn that Miranda wants to transform herself into Kat - to inhabit Kats body. To live in Kats body. After the opening credits, Kat and Rachel have an argument and Derek tries to persuade Kat to give up magic - she disobeys, obviously. After a ritual between Miranda and Kat, Miranda tricks Kat and Miranda transfers her soul into Kats body - but before she does, Miranda reveals her true self - elderly, dying old women who has been searching for the perfect body to transfer into. Miranda is now in control of Kat.