by Gary Ewer, from “The Essential Secrets of Songwriting” website:
Many songwriters are not aware of this, but chord progressions, as such, are not protected by copyright, and can be used by other songwriters. This makes sense, because if you had to come up with a unique progression that the world had never heard before for each song you write, you’d be out of songs very quickly.
It is a very useful songwriting exercise to take the chord progression of a famous song, and then apply a new rhythmic pattern, tempo and/or time signature to see what else can be done with it. You’d be very surprised to know how often this happens. Did you know that the famous musical theatre ballad “Hey There” from “The Pajama Game” follows, for a while, the chords and even the melody for Mozart’s Piano Sonata in C Major?
You won’t be able to take copyright melodies and doctor them for your own needs, (unless you do a considerable amount of doctoring!) but try this: Find a song that you really like, extract the chord progression, and then try playing it using a completely new tempo and new basic rhythm. To use the same chords with the same rhythm as the song you found it in starts to move into the copyright infingement area. So be sure that your use of the progression is unique.
This type of borrowing works better for songs that use standard progressions. The more unique a progression, the harder it is to hide where you got it.
This is a completely legal use of another chord progression. You can’t do this with another songwriter’s melodies: they are subject to copyright, and are protected from other people “borrowing” them. And infingement is essentially a cumulative thing: borrowing chords is legal, but borrowing the rhythms, instrumentation, and any other identifiable aspect of a song starts to look like stealing. Just be careful.