But judging from your example, the authorized patent must be a complete technical scheme, so the technical point from B to C must have passed by an existing technology in the authorized patent, or be considered as an existing technology by the examiner, otherwise it is not a complete technical scheme; Then, the utility model you want to apply for is either a substitute for the original technical point, or a further extension and enrichment of the original technical point; If it is the first one, as long as it meets three characteristics; If it is the second type, although the specific low position can break the high position, it depends on how you write it, otherwise you will receive review opinions.
I suggest that if it is the second case, and B to C can form a complete technical scheme, you can just write B to C.