“The Change-Up” begins with a poop joke. And not just any poop joke — this is projectile poop from a baby girl into her daddy’s mouth during a bleary-eyed, middle-of-the-night diaper change.

You’d think that would be a frightening harbinger of what’s to come over the next two hours, but it’s not — which makes it even more frustrating when you realize that it didn’t need to startle us off the top in such crass and obvious fashion. There was potential here.

When you’ve got Ryan Reynolds and Jason Bateman — two masters of deadpan improvisational comedy — bouncing off each other, you should theoretically just be able to let the cameras roll and follow them wherever they take you. With a screenplay from Jon Lucas and Scott Moore, who wrote “The Hangover” — the clever, original “Hangover,” that is, and not the lazy sequel — you should already be in pretty good shape.

But the overlong film from David Dobkin (who also directed “Wedding Crashers” and, unfortunately, “Fred Claus”) is all over the place in tone, veering awkwardly from some daring comic moments to feel-good sappiness and back again in hopes of redeeming some semblance of edginess.

Learning lessons is what body-swapping movies are all about. The grass is always greener on the other side until you have to mow it, or something. Here, Bateman plays Dave Lockwood, a wealthy, successful Atlanta lawyer. He’s happily married to Jamie (Leslie Mann), with whom he has a sweet, 5-year-old daughter and a couple of adorable, 2-year-old twins. But he misses spontaneity and fun.

His childhood best friend, Reynolds’ defiantly single Mitch Planko, spends his days doing bong hits in his man cave. Sometimes, he goes out on auditions for commercials. At night, he beds as many random women as possible. But he longs to have someone to come home to, to feel loved and secure.

One night, after too many drinks, Dave and Mitch relieve themselves in a fountain. Each insists the other guy has the better life. When they wake up the next morning, poof! They’ve switched bodies, which leads to some predictable but amusingly executed fish-out-of-water scenarios. Mitch, as Dave, uses sheer bravado to navigate the uptight law firm where Dave is on the verge of making partner. Dave, as Mitch, gets an unexpected booty call late one night from one of Mitch’s wild flings.

Olivia Wilde is sexy and charismatic — no surprise there — as the law associate Dave secretly lusts after and gets to go on a date with inside Mitch’s body. It’s nice to see “The Change-Up” flesh her character out and give her some brains and personality, rather than letting her be eye candy as so many male-dominated comedies might have.

It’s enough to make you wish the rest of the film had taken the high road. We won’t trouble you with a description of what happens to Mann’s character after too much Thai food.