Upon further reflection, I think I was much too hasty in my discussion here. You said that "Compare this to a person who is in a coma. He currently has no preferences". How do we know the person in the coma has no pereferences?

I'm going to agree that if the person has no preferences, then there is nothing normatively significant about that person. This means we don't have to turn the robot on, we don't have to resurrect dead people, we don't have to oppose all abortion, and we don't have to have as much procreative sex as possible.

On this further reflection, I'm confused as to what your objection is or how it makes life extension and replacement even. As the original comment says, life extension satisfies existing preferences whereas replacement does not, because no such preferences exist.