Melody, and of course, lyrics can be copyrighted.
Chord changes by themselves cannot be covered by copyright, thank God, or there would be very few copyrights to be had by now. An arrangement that consists of melody and lyrics, plus chord changes underlying them as a whole, can be copyrighted.
For example, you can pen new and entirely different lyrics to "Happy Birthday to You" melody, but you would infringe on the copyright holder of that song if you attempted to sing your lyrics to their copyrighted melody. You also couldn't use their lyrics with your melody without infringing.
--Mac