My initial response is that your change of direction, given it is within the same general field, would be acceptable. For history PHDs I have known people accepted with extremely general research topics, in the knowledge this would be refined to a specific area once the course had begun.
Ultimately, it comes down to whether your supervisor would still be well equipped to oversee your new research, however small the change. I would guess this would usually not be an issue. I would suggest emailing in a positive, as opposed to negative, fashion. Something along the lines of suggesting you have been proactive, and believe more fruitful and necessary research could be undertaken in option b, instead of option a.
Best of luck.