[lnanek2]

Ideally, with good encapsulation and individual files keeping to single responsibilities, you would more often get away with just changing implementation and not changing API. Although you could take the size of the change into account to penalize API changes in other files less than implementation changes in both files and ignore header files if you didn't want to clump consumer changes into the same code smell. If the API has to change, you may have exposed too much implementation to the consumers.