Something can be a unique lifeform and still change. A yeast cell is a unique organism, even though it might eventually split to form two lifeforms. A bacteria is a unique lifeform, even though it might later go and fuse with another bacteria. Obviously, then, 'what it is' will change. But that's all just labeling. An embryo is a unique organism. Heck, even if the DNA is not compatible, the organism is still alive (until it dies).
It's all labeling, but I don't see a reason why we cannot say that 'fusing' is an important metaphysical event.
What do you mean then by unique? Different DNA?
I think the existence of identical twins and chimeras really messes up any system that labels conception as the inception of an individual. All we can really say is that at some point between the sperm and ovum getting jiggy with it and the resultant birth(s) nine months later this new individual has come into being.