A RefactoringBoundary.

Is changing the interface of part of the code a refactoring?

The answer to this question is pretty simple - changing an interface is a refactoring providing you change all the callers too. A great example of this is Rename Method, which is an interface changing refactoring present on pretty much all refactoring tools.

The changing of all the callers is an essential part of this refactoring. Just changing an interface declaration will break the system - and thus isn't a behavior preserving change.

Interface changing refactorings do assume that you can get hold of all the callers, which is why you have to be much more careful with PublishedInterfaces. With a published interface, the interface itself is part of the observable behavior of the system.

Dynamic languages can make these changes much more awkward. Static typing really does help here in pinning down exactly which interface is being called at various points. Reflective calls that can also make it harder to find, either by embedding method names in strings or even composing them at run-time. This is another area where tests are essential even in environments that have refactoring tools.