Second Life is Beautiful. I knew what it was about going into it, but it’s so easy to get swept up in the romanticism and charm of the first part. About halfway through I remember thinking, ‘‘Shit, I totally forgot this was about the Holocaust.’’ That’s part of the magic of the film, I think. The characters are people before they are victims.
In the spirit of The Descent: I recently saw French film Vertige (English title High Lane), a flick that begins rather like a mountain climbing against-the-elements thriller but ends up a pretty bog-standard slasher. That sort of transition — friends get together for some activity, more or less the filmmaker’s excuse for their isolation — might not be too outside of the box for the horror genre; this takes the first act perhaps a bit further than usual.
You can catch it on Netflix streaming but I wouldn’t recommend it. It’s not a very good movie.
The Match Factory Girl (1990). The first half is completely depressing, but then it quickly turns around. Recommended.
Heathers might well be my favorite movie if they’d kept it on course, but after about an hour of pitch-black humor, it suddenly grows a conscience and gets practically accusatory toward the viewer for laughing at the first two thirds.
It is one of my all time horror movie favourite moments. I love watching the film with people that have never seen it before. Brilliantly done.
This sounds kind of like the first movie I thought of - Everything is Illuminated. It starts off as a goofy, maybe insensitive but hilarious, comedy about cultural misunderstandings. And ends up very heavy. This one is foreshadowed the whole way through so I think it counts as a gradual change in tone. (It’s much less gradual in the book; the movie cut out the half that consists of flashbacks.)
And this one reminded me of another example: Zombieland. This one starts out as a silly, gory zombie comedy and ends up being first a feel-good road trip movie with a side of rom-com (the undercurrent of black humor keeps it from descending too far into the saccharine) and then has an action/adventure ending.
This long, and nobody’s mentioned Barton Fink? It starts as a comedy about Hollywood, and gets… darker.
Million Dollar Baby. Starts off fairly lighthearted and then about halfway through turns abruptly tragic. I was sobbing by the end. (Note to self: Clint Eastwood just doesn’t DO films that are only lighthearted; what did you expect?!)
Hey, nice user name. You should have come in and mentioned It’s a Wonderful Life before I did on the previous page.
To paraphrase Ebert’s review of LITTLE BLACK BOOK, Brittany Murphy is so earnest in showing us a cute and quirky everygirl’s first day on the job – interspersed with light comedy opposite her amiable boyfriend – that we’re genuinely surprised as the film becomes a cynical and complex look at amorality.
The Brothers Bloom was pretty light and funny most of the time. The ending was a total downer.
Not a total fit, but The Dirty Dozen goes from an Act I grim beginning (a witnessed hanging) to a lighter Act II (officer whipping misfits into an effective military team) and ends with an utterly dark Act IV slaughter.
But out of nowhere Act III is a war game played for laughs that wouldn’t be out of place in a Hogan’s Heroes episode. 
Successful Alcoholics, a short film starring T.J. Miller and Lizzy Caplan, starts off as a pretty light hearted comedy about two drunks. Doesn’t quite end that way.
An old one - They Drive By Night starts off as a slice of social realism, exposing the trials and tribulations of the America’s truckers in the pre-Teamster era. Then it takes a turn into film noir before ending in courtroom melodrama, complete with scenery chewing “Mad, mad, I’ll show them who’s mad!” turn by Ida Lupino. Very odd.
I came in here to mention Life is Beautiful, but since that’s been done.
50 First Dates starts out as the kind of movie that you’d expect Rob Schneider to be in – and indeed, he does fail to add anything of value – but it ends as a pretty decent romance story.
Witness starts out as sort of an action movie, but turn into a love story. Then turns back into an action movie again.
Devil’s Advocate. It starts out as pure John Grisham: young lawyer with perfect wife is overjoyed to be hired by super-fancy law firm, enjoys the life of luxury, then slowly starts to worry that his superiors are into something dodgy, and then all of a sudden
there are succubi (or something. It’s been a while) all over the place and Al Pacino is the devil, and it’s some kind of horror film.
WTF?
The Apartment. It’s a silly little comedy about a schlub being taken advantage of by his boss… and then around the middle of the movie MacLaine’s character attempts suicide and the silliness drains out of the movie.
Kind of obscure, but the 2003 Brazilian film The Man Who Copied shifts from a quirky romantic comedy to a heist caper that involves at least one gruesome death. This shift seemed pretty abrupt to me as I watched half the movie on DVD one day and the other half the next, but the financial crime element is actually present from pretty early on – the protagonist works in a copy shop and realizes he can use the machines to counterfeit money (hence the title).
Chasing Amy. The beginning is hilarious and the ending is quite serious. Although I wouldn’t exactly say it was a gradual shift in tone.
Heavenly Creatures.
I know it’s based on a true story, and there are flashes of the end at the start of the film, but it goes from a fairly light coming-of-age tale to one of the most horrific endings I’ve ever seen in a movie.