This whole discussion becomes yet another of many red herrings that distract away from the actual point of the debate.
You specifically stated that A = B, and B = C, but A =/= C. If we ignore the inherent absurdity of this claim, which is a violation of the law of identity, and then we assume a linear move from A to C in time, then, by definition, the species officially changes when B turns into C.
In reality, if A and B and C are all distinct enough to be counted as different letters, then we would just say they are each distinct species.
If we properly define the species with enough specificity, then we can certainly point to the exact moment of the shift. As I’ve said, this is a problem of lacking precise definitions of words.