Consider someone adopted by Jewish parents, whom he grew up calling 'mom' and 'dad'. After he reaches adulthood, he meets his biological parents. Can he call them by their first names, if 'mom' and 'dad' feel uncomfortable? I know in general one shouldn't call one's parents by their first names.
The Nishmas Avraham (Chelek Hey Choshen Mishpat Siman 42) and Yalkut Yosef (Kibbud Av Va'em Perek Vov Sif Vov) (among others), rule that an adopted child is obligated in honoring his biological parents. Although there might have been room to say that giving their son up for adoption is considered mechila (release) of their right to be honored, they both refute this idea.