Same tune, different chords. A reharmonization keeps the melody but swaps the harmony under it for something richer or more surprising. The staples: the tritone sub (a dominant replaced by the one a tritone away — it shares the guide tones), secondary dominants (make a chord the V7 of the next), the backdoor ii–V (reach I from ♭VII7), and modal interchange (borrow a chord from the parallel minor). Start from a I–vi–ii–V turnaround, apply a technique, and A/B the before and after.