Well, the answer is a lot less scandalous than I thought.
Apparently, the reason why the title is different in the U.S. is that another book called The Seven Husband of Evelyn Hugo was being released right around the same time, and publishers wanted the book to not be confused with the very similar title of The Seven Deaths of Evelyn Hardcastle. As Stuart Turton stated in an interview,
The US title is different because it came out after a book called The Seven Husbands of Evelyn Hugo and we were trying to avoid too much confusion. Our plan hasn’t entirely worked, to be honest! I get so many people telling me they bought one, meaning to buy other. Some people think 7 ½ Deaths is a sequel or a prequel to Seven Deaths, or Seven Husbands. -LAPL Blog
The plot doesn't get changed, and it's the exact same book.
Now of course, for those of y'all that are really curious on why it's still called Seven 7-and-a-Half Deaths... well read the book, major spoilers spoiler warning.
Here's the synopsis, according to Goodreads:
"Tonight, Evelyn Hardcastle will be killed... again. It is meant to be a celebration but it ends in tragedy. As fireworks explode overhead, Evelyn Hardcastle, the young and beautiful daughter of the house, is killed. But Evelyn will not die just once. Until Aiden – one of the guests summoned to Blackheath for the party – can solve her murder, the day will repeat itself, over and over again. Every time ending with the fateful pistol shot. The only way to break this cycle is to identify the killer. But each time the day begins again, Aiden wakes in the body of a different guest. And someone is determined to prevent him ever escaping Blackheath..." Goodreads.
Now here's the catch (spoiler warning):
Aiden has eight hosts he can inhabit. He stops Evelyn Hardcastle from dying on the seventh day and "solves" the murder on the eight-day (well he's technically right but like he's not). Except... that wasn't actually Evelyn that's been dying. It was someone the real Evelyn hired to take her place because Evelyn Hardcastle was trying to fake her death. The real Evelyn, in the most dramatic twist actually tries to kill Aiden and his friend Anna, and the fake Evelyn (whose name you'll find out) kills the real one, saving Aiden. So the assumption is that there are 6 deaths of the fake Evelyn, and the final death of Evelyn, making it seven, while in the U.S. version, it can be seen as the seventh day (where the fake Evelyn almost dies), being the half death.
Now, go read the book!