I just finalized the adoption of my son and still call him by his first name but have altered it a bit on the birth certificate to a more formal version of that name. I changed his middle name totally and of course he gets my last name. He is only 3.5 years old now and I figure that if he wants to he can change his name back when he's older but since his first name is the same I don't think there will be any problems for him.
I have a mediation agreement with his bios that they get 4 visits every year (so far the bmom has missed 2 and the bfather 1). And they call him by his first name but I'm sure he's going to blurt out his full name at the next visit (Nov.). He likes to tell people his full name...
I think it should be left up to the adoptive parents. It's a new beginning for him and for me and I think it deserved a fresh start. He will always know about his birth family and I hope he doesn't mind that his name was changed. It's up to him, who knows what the future holds...