IGoing

I was playing DayZ with my friend's account, but now I bought it on the Steam sale. Although his account is still attached to my game and when I'm playing he can't and vice-versa. I already reinstalled the game and tried to change the CD key by launching the game from my Steam account but nothing happened. If I log on a server he is playing I get kicked and if log in another one I get stuck in "Waiting for host". Is there another way to change the CD key?