Copyright Update: Difference between "Regexp to detect" and "Allow replacing"?

I'm trying to use the Copyright plugin for IntelliJ and I'm confused by two of the fields. There are the fields "Regexp to detect copyright in comments" and "Allow replacing copyright if old copyright matches". I don't understand the difference between the two things and in what ways you'd use them together. Both of these fields seem to be used to identify which copyright blocks can be updated with the text above. Can someone explain? The documentation doesn't really seem to make much of a distinction between them.

Here's the current documentation: https://www.jetbrains.com/help/idea/2018.3/copyright-profiles.html?utm_campaign=IU&utm_content=2018.3&utm_medium=link&utm_source=product