Upgrade client to pro

Hi there,
I have a client that I manually gave a pro membership to while I was developing the site. Now I want to upgrade them to a paid version. I switched them to a 'Pro Bono' level, which is free, because I don't want them to have any down time. Now, when I try to upgrade them to pro, it's only giving me the option to create a new site. Is there any way to do this?