Eb has transitioned from “Sir”, to “Wizard McCoy”, Harry doesn’t believe he now needs to show Eb any deference, either as a former Master or as his grandfather. That is twisting the Athame, it indicates a permanent break between the two.
Margaret was being trained as his replacement, she rebelled, Harry wouldn’t accept it other than pass it to Mother Winter, which would get him a favour, just when he needs one for Thomas.