Or was there some "magical" aspect? The transformation is of course quite remarkable but also, when the main character, pre-transformation, says he wants to be a tennis pro, he is told this would not be possible and I think he decides to become an artist. So perhaps this implies that the the transformation is effected surgically and maybe involves, in the 1960s, hormone treatments or something.
A very creepy movie, I recommend it indeed.
I think it's purely cosmetic surgery. Whether there are additional drugs or procedures is not clear, but there seemed to be no paranormal or supernatural elements involved. I would say the closest approximation would be the creation of a deep cover story for an agent which would defy even close scrutiny. The tennis pro story was probably impractical since he was a late middle aged man who didn't appear to have the physique or skills to pull off the illusion. Since the actual source of his funds was the results of the large life insurance pay off from his purported death, his after-life job is largely a sinecure created for show.
