RE: What are keywords?

As with any regression testing, changes to code and functionality can affect previous results. That is the entire point of regression testing. If changes never affected existing results, then there would be no point to regression testing.

New responses will affect existing "non exact" responses if they are better matches than the existing response. By doing regression testing you will be able to review existing questions that are affected, and if they new responses are not correct then fix them accordingly (by adding the questions to the intent label, or by configuring keywords correctly).