I'm somewhat ashamed to confess I first became aware of the existence of this work courtesy of the film version of Amadeus wherein was heard part of the meltingly beautiful third-movement Adagio (at the point in the film when we first meet Mozart as musician) which, in the film, came to such a horrific, jarringly wrong close musically that I simply couldn't believe Mozart actually wrote it that way; ergo, my acquisition of the CD of the full seven-movement Serenade.
Turns out, of course, that Mozart didn't write it that way. What Sir Neville Marriner (Amadeus's music director) did for the film was to take the close of the seventh movement Molto Allegro Finale of the Serenade, and tack it onto the third-movement Adagio to form its close. Whatever possessed him to perpetrate such an idiot and grotesque edit is simply beyond my meager capacity to imagine, but, to quote the film's Emperor Joseph II, "There it is."
I may be wrong here, as it's been a while since I've seen the film (too long, actually, since I think that it's a great, great film), but I think that the musical edit here isn't intended to change the music itself at all. Rather, it is necessitated by the film itself. All that happens is that the film simply cuts from the middle of the work (that sublime movement that Salieri describes vividly) to the end, so we can get on with the business of Mozart being chewed out by the Archbishop (if my memory of the scene is correct). I don't think that Sir Neville Marriner meant to imply that the brisk conclusion of the Serenade's finale occurs in the same movement as that wondrous Adagio: he is telling us, "OK, we're at the very end of that same concert now."
A similar cut from one part of one work to another, later on the same performance, happens later during the premiere performance of Le Nozze di Figaro, when we go from one moment in the opera to another moment later on (when the Emperor starts getting fidgetty and yawning). That's all that's happening here: the film jumps from one point to another. Thus the musically "jarring" edit, which is probably jarring by design: Marriner very likely needed to make sure that it was blatantly clear to the film's audiences that cuts like these had taken place.
(By the way, I count the soundtrack to Amadeus as an exception to my rule of never recommending that people buy "classical sampler" albums consisting of single movements of various multi-movement works. The Amadeus soundtrack, on two CDs, is a very-well considered product, so if you've just gotta buy a "Mozart Sampler", this is the one to get. And the only one to get. Otherwise, buckle down and listen to complete symphonies, concertos, operas, and so on.)