Inspired by this thread about movies.
What songs can you think of that change at some point to what could be a completely different song?
A famous example is the Beatles’ A Day In The Life, with John’s “I read the news today, oh boy” part morphing into Paul’s “Woke up, got out of bed” segment and back again.
I can think of others, but I’ll throw it out to everybody else for more.
“Layla” by Derek & the Dominos. A guitar rocker with vocals that turns into a wistful piano solo with some guitar backing.
I’m not sure if “We Will Rock You/We Are The Champions” counts, since it often gets broken up into two separate songs for radio play.
Long John Baldry -* Don’t try to Lay No Boogie-Woogie on The King Of Rock n’ Roll*
First half funny little story, second half rockin' Boogie tuneIn some cases it can be difficult to tell the difference between this and a two-song medley, if indeed there is any difference.
But I have seen
You are not listening
Poems that are structured
When I scream at you
In such a way
That I am not listening
That when you hear them
And I am going
You will find out
Away.
Are any of those ones well-known songs?
The Guess Who put “No Sugar Tonight” and “New Mother Nature” together at least sometimes, if not all the time.
“The Chain” by Fleetwood Mac has to be a perfect example of this.
Songs that are two songs put together.
Songs that change completely (and stay that way) somewhere in the song.
The story behind that is that they had two songs that were in the same key but weren’t long enough to release as singles, so they simply morphed them together.
And then there was this. Watch at your own risk.
I think most progressive rock bands can get at least five songs into one 
I knew I remembered this from somewhere!
Sanglots - poem by Guillaume Apollinaire
Scroll to 5:22 for Sanglots - song by Francis Poulenc - Poulenc himself at the piano, Pierre Bernac singing.
A different kind of two songs in one.
Ike and Tina Turner’s version of Proud Mary:
Billy Joel’s Scenes From An Italian Restaurant. One song about a couple in a restaurant and another song about Brenda and Eddie.
That would be Brender.
“We’re Not Gonna Take It” by the Who, whose second part is also known on its own as “See Me, Feel Me.” But it’s a single song.
Isn’t that how it’s pronounced? Brender’n Eddie. They be sipping gin and soder, With lil Antnee an’em. That’s how we talk on Lawn Guyland.
Freebird.
(raises lighter)
Paul McCartney’s “Uncle Albert” is three different songs in one. Different tempos, different melodies, different voice inflections. But that’s nothing new for Paul, of course.
Aquarius/Let the Sunshine In from Hair
Genesis’ “Supper’s Ready” for example, which has 7 parts (and is 23 minutes long)