Wendy Huang
Wendy Huang thanked Cookie Roma's answer

The adoptive parents would have to go to court and give up their parental rights.  Then the biological parents would have to adopt her.

Wendy Huang
Wendy Huang thanked Anonymous' answer

I did find a really good article on what is called "reverse adoption " and if you read further down, there are several conditions which a judge will grant this and revert the child back to the biological parents.