I keep coming across this situation and was hoping someone would have more information on this:

We have an existing product and I have been requested to "make something work better", so I start by considering what the flow/screen should do and looking at existing design patterns for the functionality. I Come up with a wireframe of how the user flow/screen can be represented and show to the stakeholders. Most people agree that the changes are improving the experience but then someone raises the issue: 

"what about existing users of the product they will now be presented with a new screen and it wont make sense to them."

Now ideally I would suggest testing it with some users, but when this isn't possible I have provided evidence that the approach I have taken uses existing patterns that are common in web applications, so therefore users should be able to understand how to use them.

So I was wondering can you think of anything else I can do? Or do people think we shouldn't change things as it will effect existing users too much?