Unit tests are most often used to cover a few more lines that need coverage. That's the value they provide.
You could do it in Word instead, I suppose, but if you write it in code then a computer can validate that the documentation you wrote is true. That brings tremendous value.
That’s assuming change is even necessary. Oftentimes you only need to modify the implementation, which doesn’t change what is defined. A change in implementation has no impact on the outside, at least as long as you have properly covered your bases, which should you should be able to do as long as you have proper documentation. Without documentation, good luck to you.