I was listening to WH’s l Will Always Love You thinking about how cool not only that modulation is (3:05 below) but how nice most modulations are. But then it dawned on me that the only examples I can remember always modulate up. Do any popular songs modulate down?
I don’t know any off hand, but that’s no surprise because I don’t know popular songs that extensively. But I suspect you’d start your hunt for downward modulations mostly in depressing sad songs.
“Penny Lane” from verse-chorus goes down a whole step from B-A - but tricks you, because the melody line goes up to reach its highest note.
My musician friends and I would joke about the “gearshift” modulations that seemed to be the spate of music in the 80s. Those are usually cliche and annoying, at least to me. (Looks like it’s also called a “truck driver modulation,” according to a cursory internet search.) It’s basically when on the final chorus, running out of room and creativity to ramp up the song’s energy, the song modulated up a semitone or a whole tone, sometimes multiple times. I think we called it a “gearshift modulation” because it sounded like someone shifting down a gear (higher RPM, higher pitch.)
Of the top of my head, I really can’t think of much. Derek and the Domino’s “Layla” starts in D minor for the intro, then moves to C# minor when the verse starts. But it bounces back up to D minor for the chorus, and back down to C# minor for the verses.
I’ve always thought it was called the “truck driver’s gearshift” modulation because it was so clunky sounding - no relative minor, pivot chord, etc - just shove the lever up a notch.
Thanks for the heads up on Penny Lane. Googling, I found this: Deconstructing Penny Lane - YouTube
I’ve never given that song much thought but now I have a new appreciation.
If you are interested in popular songwriting, you can’t go wrong with studying The Beatles - there are numerous books, websites, etc. dedicated to breaking down their music.
That makes more sense. I never really thought about it until having to try to explain it here. It just always sounded like the right term for it.
Johnny Cash’s “I Walk The Line” modulates down to the point that it depends on how the singer’s voice is doing on a given day that it works.
Do you mean the song is variable length, and continues until that particular singer can’t reasonably go any higher or lower?
The most famous is Conway Twitty
I’d Love To Lay You Down
A very innovative way to wind down the song. Goes well with the lyrics as the character ages.
Modulates down three times
It’s been a few years since I performed I’d love To Lay You Down
IIRC it starts in D
goes down to C and then down to B flat
Tricky to play solo because you can’t use a capo. Gotta man up and use barre chords to play in Bb.
Years ago I saw a band perform I’d Love to Lay You Down that repeated the final chorus until the lead singer was singing full two octaves below where they started.
That would be really cool. 
Singing that low is hard, but very effective. It’s the characters dying breath, declaring his eternal love for his wife.
I’ve always loved this song.
Johnny Cash’s rendition of ‘Wabash Cannonball’ modulates down 2 steps right at the beginning. My best guess is that it imitates a train’s horn or whistle modulating downward due to the doppler effect as it passes.