I think I'm starting to understand what you are saying. The key term that is still sound vague for me is interface. My understanding of it is: interface is a small application that initially uses the code that I want to modify, which then changed to use the new code when the new code is ready. Hmmm. So I need to tests the behaviour of the interface instead of the code I want to change. Is that correct?
This is Brilliant!!! Pure Genius!!! This way I can be 99% sure that the new code behaves like the old code and that confirmation comes from the exact same test while I can significantly change the code. Thank you GrandFather. I wouldn't think of it myself. ++ for you. I guess I have to read up a lot more about refactoring technique.