As the beginning of the new Ghostbuster's Generation, an ancient evil spirit called Achira has been freed in New York. Looking for a willing host to infect the whole city with her terrible sickness, she founds one in Kylie (one of the new Ghostbusters in the next episodes), who mistakes Achira with her deceased Grandma Rose. As result, Achiras merges with Kylie and possess her body. With this, Achira switch her appearance from Kylie's human form to the form of an adult woman dressed as a witch. The transformation looks like a "Age progression" where Kylie turns into adult.
In this clip of the episode is when Achira merges with Kylie and when Achira switch Kylie's human form to witch form: