To its credit, The Change-Up lets you know exactly what kind of movie it’s going to be in the very first scene — within the first 90 seconds, the movie announces loud and clear exactly how awful it will be (very) and thoughtfully gives viewers time to flee the theater.

That is the one and only thing it does right, a brief concession to the audience before it launches into a tirade of some of the worst filmmaking seen this year – or any year, for that matter.

The movie opens with Dave (Jason Bateman, Horrible Bosses) waking up bleary-eyed to the sound of his baby screaming. He’s clearly miserable. With a loving wife (Leslie Mann, Rio), a beautiful home and a successful career, who wouldn’t be?

He goes to change one of his twin sons, providing an opportunity for the first of many close-ups of baby-ass, a close-up that’s held even as the baby farts and then projectile-poops into Bateman’s mouth. Within the first 90 seconds.

Thus, a cinema classic is born.

He works as a lawyer (although he spends a lot of time negotiating mergers, and not just from the legal end — he apparently does all the financial work too, because hey, why not) where he has the same sneering boss every other Hollywood white-collar guy has. “A double Windsor? This isn’t the dog track,” he says to Bateman, as well as, “You look like a Jew.”

He also has the expected sultry coworker (Olivia Wilde, Cowboys & Aliens) he secretly fantasizes about — and, surprise, it later turns out she has a crush on him, because he is the main character, so of course she does.

She is, like every other character, a completely over-the-top cliché. She’s the powerful, sexually voracious woman who gets her nether regions tattooed on first dates and whose only purpose is to tempt the protagonist with her Jezebel ways. The film’s gender politics are, politely, stone-aged.

The foil to Bateman’s emasculated, overworked, weakling of a man (because all successful family men are emasculated, overworked weaklings in Hollywood) is Mitch (Ryan Reynolds, Green Lantern), a high-school dropout who works in porn and spends his days stoned and his nights sleeping with beautiful women, because uneducated, underemployed soft-core porn actors are apparently catnip to desirable women in this parallel universe.

Each wants the other’s life. And wouldn’t you know it, after they drunkenly urinate in a public fountain, they wake up the next morning to find their wish has come true!

After the “Oh my god, I’m in the wrong body!” scene and realizing their drunken urination/wish-making caused the switch — a conclusion neither seems to find very surprising — the two try to find the fountain (which was relocated, because duh, plot complication) and undo the change while attempting to not screw up the other’s life.

Neither is very good at that. Dave-in-Mitch (Reynolds) spends his time learning to rollerblade — married guys can’t do that, for some reason — and reading Jonathan Franzen’s Freedom on the toilet, with all the hee-larious fart noises that implies.

That’s harmless. Mitch-in-Dave (Bateman), on the other hand, sets about systematically destroying his best friend’s existence. He corrupts every facet of his life, wrecking his career and undermining his marriage. “I’m not attracted to you,” he tells Mann, moments after ogling her.

It’s thoroughly unpleasant to watch, and not in an amusing, Larry David-type way. He’s just a hateful character, one that you want to see receive a major karmic kick to the head. Whatever goodwill Bateman had left over from Arrested Development is squandered here.

The film mines every source of obvious, lowbrow comedy it can think of. There’s Asian stereotyping (“These squids, these Japs, these Kamikaze pilots,” Bateman says, for laughs), gay panic, baby endangerment, shitting, farting, pissing and every other type of vulgarity. Calling it lowest common denominator is giving it too much credit.

All of this makes the inevitable, insincerely sentimental closing all the more insufferable. You want to see them fail. They deserve to fail.

But, of course, Mann continues to love Bateman — even as he grossly mistreats her, he’s able to swear and curse his way through the merger, and Reynolds gets to give his big redemptive speech right before the switch-back, which begins with “There’s a girl scout staring at my penis” and ends with him peeing on Bateman’s leg.

This is the kind of film where Mitch is seriously compared to Robert Frost, where there are more breasts than a Russ Meyer film, where there are no less than three bad montages, and where characters weigh in on the ongoing philosophical debate of whether it’s gay to jack off in a body-swap comedy. Avoid it like the plague.

RATING: .5 stars out of 5

diversions@umdbk.com