In web service, how to change the interface without updating the client?

In the book 'Java web services' by Chappell and Jewell, it describes loosely coupled being one of the characteristics of web services. It went on to state the following:

"The web service interface can change over time without compromising the client's ability to interact with the service."

My question is, how can the web service interface change without changing the client? From a coding perspective, if an interface change (new parameter, new name etc..), the caller needs to update the code accordingly. Can someone help me interpret the above statement?