Fancy Lala, the Magic Stage
Ep12 "Who Are You?" 1998/06/21 - "Anata wa Daare?"
The only time that Fancy Lala was shown reverting back into Miho:
Ep14 "Miho in Toyland!" 1998/07/05 - "Omocha no Kuni no Miho"
Aborted grow-up scene. Miho attempted to age herself into Fancy Lala, but immediately reverted back. Her magical adult clothes also vanished when the transformation process failed:
Miho was reverted from her real age (9 y.o.) into a preschooler when she entered the magical Land of Toys in her dreams. Her older sister was also briefly sucked in to the dream universe: