My client had asked for work on a small book. Later he asked for a new chapter, beyond the changes we originally discussed. I allowed that to slide along.
Then he asked for the new chapter to be spun off as a separate article. Again, I agreed, because the chapter already was fairly self-contained.
When he asked me to re-work the article, in order to convey a different perspective entirely, I pointed out that we were experiencing "scope creep" and asked how the payment would square with the work.
The good thing is that he agreed with my analysis and offered extra pay. The unfortunate thing is that we should have preemptively dealt with this issue in the original contract.
The business TIP: Agree on how to deal with changes before they are requested. Ensure that it will be fair to both parties.