Since the dawn of language there have been stories about transformations. And from Ovid to Kafka, Western European literature has been fascinated with the literal and figurative change from humans to animals. Cinema has continued this tradition from the early years, when smoke and mirrors transferred from stage to screen to provide the effects combined with old fashioned acting skills; John Barrymore is convincing in DR JEKYLL AND MR HYDE (1920) contorting his face in absolute agony, for example.
The early 80s marked a Golden Age of prosthetic effects deployed to create some staggering scenes. It was a time that included THE THING and AN AMERICAN WEREWOLF IN LONDON, where the effects managed to create shocking transformations in a genuinely visceral manner. Prior to this time, the changes were gentle cross-dissolves, after this time they have become matter-of-fact cgi.
Here are some of my favourite transformations and one change for the worse.
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1) THE FLY (1986) The creepiest moment remains big-faced Gina Davies attempting to clip a stiff hair on Jeff Goldblum’s scarred back. Urrggh! Watching it now, it isn’t as good as I remembered it.
2) THE COMPANY OF WOLVES (1984) The young groom stripping his flesh off before his young bride, because the young girls should beware of those who are furry on the inside. Urrggh! Watching it now, it isn’t as good as I remembered it.
3) TERMINATOR (1984) Possibly the longest change in movie history. The skull beneath the skin is revealed as Arnie is a unstoppable force meeting a the movie-ble object of time paradox.
4) SINBAD OF THE EYE OF THE TIGER (1977) – A film that features a couple of stop-motion change spells but the best is when Zenobia turns into a seagull to pester Sinbad and loses some of the potion to change her back. For the rest of the film she limps along with an ugly seagull foot (which is ridiculously exaggerated on the poster).
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5) TRANSFORMERS 2: REVENGE OF THE FALLEN (2009) – if evidence was needed of how eye-popping CGI can render the spectacular boring, then watch these robots hitting each other for 150 minutes.
The Dirk Malcolm Alternative 
There could be lots of Cronenberg moments in this list of course, notably the stomach bit in ‘Videodrome’, but ‘The Fly’ is the film of his I find myself returning to most often. Although it might be the inside-out baboon I’d choose.
How about the fat woman in ‘Total Recall’ turning into Arnie? I think I’d rather become a giant bug.