A popular song (or even a nearly unknown one) has a cover version released. Rather than stick to the tone of the original, the band covering it decides to take it a different way. If the original was a happy song, it may be redone sarcastically or sadly. If the song was about the breakup of a relationship, it could be changed to being about the character's Sanity Slippage. Either way, the meaning or the tone of the song is completely different in the cover version.
See also Softer and Slower Cover. Can overlap with The Cover Changes the Gender and Repurposed Pop Song. Can also overlap with In the Style of if the cover changes the tone or the style of the music as well.
Compare Dual-Meaning Chorus, where the original song is written to offer up several different interpretations of its chorus. Not to be confused with Parody Displacement, in which a parody of a song becomes more popular than the original. If the new version becomes more popular, it may result in the original being Covered Up. What Song Was This Again? is when a translation of a song in a different language changes the meaning. When done with a song meant for small children, it's an Ironic Nursery Tune. Don't confuse with Covers Always Lie (as in the physical cover).