How not to violate software copyright policy?

One of my clients asked me if i can modify an application (source code) to such extent that it will not violate the copyright policy anymore. He asked me to sign a confidentiality agreement, and then he would give me the source code.

What i would like to know is: what do i have to modify and how much of the code do i have to rewrite so that the resulting application will not violate the copyright policy of the old one, and, am i breaking any laws? or it is only my client that will break them? Thanks!