Basically in order to start you have to have a plan.
Despite knowing full well things will change, that plan is used to set the expectation.

Now if you were to be so foolish as to recognise the change in requirements by changing the plan, you will be adjudged to have failed to meet the expectation.

So what happens, we stick to the plan, but describe the process with terms gleaned from the glossary of the latest how to do agile book....