Whatever entertainment there is in The Change-Up probably comes from its adherence to the conventions of the "body swap" comedy. Its disappointments rest squarely on its misguided attempts to think outside the body-swap box.
We know the drill: Two people express a desire for a different kind of life, and paranormal means accommodate them. In this variation, Dave (Jason Bateman) is an overachieving attorney with a wife (Leslie Mann) and kids; Mitch (Ryan Reynolds) is an underachieving stoner wannabe actor who lives alone. One shared piss in a mystical fountain later, Dave's mind is waking up in Mitch's body, and vice-versa.
Bateman and Reynolds are both wisecracking, sardonic screen presences, and they both get some solid material to play with as they wreak havoc through one another's personal and professional lives. But their similarities force us to confront Body Swap Comedy Law #1: Make the switch a noticeable one. The common flip-flops of parent/kid (Freaky Friday) and male/female (All of Me) allow actors room to play with very different personalities. There's just not all that much difference between Bateman freaking out and swearing and Reynolds freaking out and swearing.
They also both get involved in some genuinely preposterous set pieces, which brings us to Body Swap Comedy Law #2: Let simple situations drive the comedy. It's already weird enough that two people have traded consciousness; the fun comes from watching them confounded by what would be another person's everyday scenarios. While it's funny watching family man Dave respond awkwardly to his first "first date" in decades, it's ridiculous watching him muddle his way through a "light porno" movie shoot.
Which instantly collides with Body Swap Comedy Law #3: Don't work too blue. There's a reason most previous incarnations on this theme have been family-friendly: The concept is a fundamentally conservative one that reminds us that, whatever their troubles, our lives are basically good the way they are. The thematic twist here isn't all that radical - both guys ultimately see where they need to improve themselves - but the content definitely is. Even when the crudeness is hilarious, it feels like an awkward fit with the sentimentality.
It's telling that most of the best moments in The Change-Up don't involve either of the two leads at all, but rather Mann's terrific bits as Dave's frustrated, oft-ignored wife. She knows how to get the most out of playing by the genre's rules, while The Change-Up makes the same mistake as most transgressors with an adolescent sense of humor: thinking that breaking laws is inherently cool.