When Does a Fix Become a Change? [Traffic Talk]

This question originated from reddit and the person asks:

When does a fix become a change?

For more context:
I develop software and clients don’t seem to ever understand that it’s not usually as easy as changing the color or something when I’m changing/adding a feature. They usually look at it as a “fix”.

I’m beginning to better document their needs beforehand to avoid this but for you when does a “fix” become a “change”?