First of all the changes do not define the song. Any tune from the Real Book, or other sources, can be harmonized by several different sets of chords, you have many options for this. The Head defines the tune. If you look at a few dozen tunes from the Real Book at random you will likely find a few common sets of changes. If all anyone did was follow the changes then every tune would sound the same and solos would be boring. Improvisation is variation on a theme. The Head is the theme and that is where you get the most bang for the buck.
That all being said, in western music changes naturally arise from the melody and application of a few simple rules. If you really understand the rules of harmony theory you can harmonize as you go. Piano players and guitarists do this all the time. With the melody in front of you you definitely can "improvise" changes. Given some knowledge of the circle progression, cycle extensions, and substitutions you could get some pretty intricate harmonization of even a simple melody. Being able to do this takes a lot of practice in my experience but it is possible.
If this is not what you were asking about, please comment and I'll edit the answer.