I'm thinking about creating a jingle for a future radio advertisement.
 One suggestion I've heard is that you take an existing copyrighted
song and change both the lyrics and structure of the chords/tune.

What are the guidelines or legal test explaining when the change is
significant enough to avoid a copyright problem?

Thanks,

Andre

If you cannot avail yourself of fair use, then you must change it
enough so that is neither a derivative work nor is it substantially
similar to the original.  These are all terms of art in copyright law.
 If a Researcher doesn't elaborate, I'll come back and flesh this out.