I don’t know if you can do this in iChat but it’s easy to do in the official AIM client. In the official AIM client, sign in on your old screen name and there is a command “Export Buddy List” which exports is as a file on your desktop. Then you could sign in on your new screen name and there is a command “Import Buddy List”. If you can’t figure out a way to do it in iChat, that’s what I suggest you do. But iChat is better overall, since it is so much faster and doesn’t have ads.