RPC/literal in Ivan notes

I am very impressed with scdjws5studynotes by Ivan.I got a doubt while i reading this,he mentioned the drawback of RPC/literal as

A drawback is that if there is a new version of an operation in the service that needs, for instance, additional parameters
then the clients need to upgrade as the service changes, unless of course both the old and the new
version of the service are published at the same time.

what it mean by upgrade as the service changes?can any one explain this very clearly?

RPC Clients must match the Web Service Providers layout (WSDL).

As this changes, it can render the client modules inoperable.

This is not the case (mostly) with document/literal SOAP conversations.

WP