I am looking for a live action film on full body transplantation. I have only very vague memory of this film, so I'm unsure about most of what I say here. I am only sure about two details that really stuck out and made this film unique.
I saw a segment of this film on TV in Hungary, probably some time between 1995 and 2006. The dialog must have been in Hungarian, but I don't recall if it was original or dubbed. I don't think I saw the entire film, so I don't know how it ends or how it starts. The film may have been from the 1980s or even older.
A mad scientist surgeon and his small team of assistants perform full body transplantation operations on humans. This was shown in a science-fiction way, with no supernatural fantasy elements. The film showed an operating room like you'd see in a hospital, where the surgeon and his team removed the brain from the patient's skull, inserted it to the host body's skull, and reconnected the nerves to the new body.
The people who gave the new host bodies were kidnapped, and operated on unwillingly.
Perhaps the patients were rich people who would pay for that illegal service. (Update. On further reflection, I think the paying customers weren't aware that they were transplanted to another person's body, they were only told that they got a miracle aging cure.) The part that really made this memorable is that the victims who provided the unwilling host bodies weren't killed, instead their brain was transplanted into great apes, and then the apes kept in a cage, presumably in the laboratory. The victims couldn't speak as apes, and so few people would suspect that they were humans inside, thus making their situation hopeless. I think that one of these victims was the protagonist of the film, male, and his lover somehow met him and recognized him even though he was in an ape's body.
The second distinctive and memorable part of the film was this. After a few successful transplantation, when the mad scientist was certain that the operation works well enough and his team is capable, the leading mad scientist himself laid on the operation table and bid his team to transplant him into a younger body. The mad scientist was probably male and in his middle ages. However, his team betrayed him, and he woke up in the frail body of a very old man.
I tried to look TVTROPES WARNING on the TvTropes page for the trope "Grand Theft Me*, which seems to be the most relevant trope for this, but nothing in the "Films — Live-Action" section seems to match.