OK, here is a link to a clip from the movie 'The Howling II: Your Sister is a Werewolf'. The movie stars Christopher Lee (who does not feature in the clip).
I have reworked this scene, heavily editing the content, to try and create some sort of basic process. In the original, we see old hag 'Stirba' become busty, horny young 'Stirba' (portrayed by Playboy actress Sybil Danning) after sacrificing a virgin bride and sucking the youth from her of course! They use cutaways to howling deciples and a strange, but unconvincing, dissolving head. All I have done is slowed it down slightly, cropped the before and after and blended them as best I could, with a simple 'Transition effect'. I think it works well. I will upload the untouched clip at a later date.
http://209.245.59.102/Gimme/86220754.../stirbamov.wmv