The preview of this refactoring needs to be able to show what each change actually is. I.e. what is the code going to look like after the refactoring is performed. Just showing the list of declarations to be changed is not sufficient on such a wide-scale refactoring.
I know that doesn't fit into the current refactoring preview UI, but in my mind this is a sufficiently different refactoring that it warrants either updating the refactoring preview to allow this sort of display or it's own special preview UI.