Let’s remove from the equation “when the soul becomes present”, let’s ask the atheists what the ethics are and when it should be considered an equal PERSON (or persons).
No reason we couldn't have folks around without souls already ~ maybe we might call them sociopaths.
Let's say we figure out what the "soul switch" is and then develop an injectible "fix" for that problem.
Would it be considered ethical or unethical to create souls in humans without souls?
(NOTE: this is like counting the number of angels dancing on the head of a pin of course, but this whole business can be flipped around and aimed right at the sociopaths who PREFER embryonic stem cells. It threatens them with their own science. Further, imagine the guilt pangs they feel after growing a soul. Another avenue of debate might be what the anti-vaccination people would do?)