ssh-add -l to check what key you are using.
ssh-add -d ~/.ssh/id_rsa
ssh-add -d ~/.ssh/github
Add the github ssh key
seems not working.
I found that I had to delete the Keychain entry for my old account and Magically the username and password prompt came back.
To delete a keychain entry on Mavericks. Pres cmd + space then type key chain. Select Keychain Access. In the new window that pops up search for GitHub. Delete both of them.