“You discover that wonderful one-year-old child is, because of a mixup at the hospital, not yours. Would you want to exchange the child to try to correct the mistake?”
I would not want to make the exchange, but I would feel compelled to do it. There isn't a great solution to this dilemma. I would hope that I would still be able to be involved in some way in the life of the child whom I raised for his first year.
I'd also like to reserve the right to trade back before age 18 if I didn't like my biological kid. (Just kidding.)