"Key Exchange" traces the romantic inclinations of two couples, one married, one not. A vanilla and uninspired romcom with a handful of nondescript actors yammering their way from one insipid scene to another, this easy-going flick has little to offer save a few humorous moments and some misfired attempts at poignancy. Too good natured not to like, "Key Exchange" makes for a mild watch for the bleary eyed sofa spud. (C)