I came across this mashup by Wax Audio on YouTube today. It mashes up the vocals and horn lines from "Superstition" by Stevie Wonder with the instrumentals (and occasional vocal interjections) from "Sad But True" by Metallica.
It begins with Metallica's introduction, pitched up one half step from its original key of G minor. Then song proper then begins in E-flat; again, up one half step from the original key of "Sad But True." Stevie Wonder's vocal line comes in, unaltered for pitch, but slowed down from its original setting.
The mashup continues with Stevie Wonder's vocals (and prominent horn lines) and Metallica's instrumentals for the majority of the track. Combining Stevie Wonder with Metallica results in a genre crossover, which audiences value. However, there is no harmonic recontextualization in this particular case. The chord progressions of the two songs are essentially the same. The only thing that Metallica's guitars are changing, essentially, is timbre. It's almost as if someone is covering "Superstition" in a heavy metal style.
The reason these two songs go so well together is not because of a surprising melodic/harmonic compatibility, but because of nearly identical harmonic content.To be clear, I don't think the lack of harmonic recontextualization makes this track any less valuable as a mashup. But harmonic recontextualization, when it does occur, adds an element of surprise to a mashup that competent listeners appreciate.