It changes when it can live on its own outside of a womb.
That would be never. None of us can live on our own. Remember what I said earlier about if our gut flora fail? We get
really sick if that happens, and we die if the balance isn't restored. We also all die if plants stop making oxygen. Also, like the unborn and just about everything known to exist save the water bear, we die in a vacuum- or anywhere else outside our natural environment.
We used to die if certain parts of us died, but now we can keep Dick Cheney alive with no heart and we can almost replace our lungs. At this rate, we might be able to replace our entire bodies within a century and we won't have to be alive at all in order to be able to have this discussion.
Also, if viability as you envision it is the guideline here, then what about when grandma's on a breathalyzer or when grandpa needs kidney dialysis? Or a baby who need to be rushed to the ER after birth? Again, none are viable- does that make it okay to just shoot them in the head and be done with it? Then we have many who never achieve viability. Others have to be robbed of their lives, in terms of their biosynthetic energy and their liberty (time) in order to have the fruits of their labor stolen to support the old, the weak, and the lazy who have no lives of their own, who can't sustain their own existence and who must suck the life and wellbeing of others like parasites in order to continue their existence.
But you never answered the question. Is it okay to kill you right now because i don't want you to exist? If not,
why not? And at what exact moment did what , exactly, change that made it no longer okay to do so? Was it when the afterbirth came out? Could I just leave you in a dumpster then or leave you on a table and let you have a life of your own if you were really alive?