I had this issue as well, but was able to get around it. On your My Sites page, I was clicking the top-right account menu and going to Premium Plans and transferring from there. The same thing happened to me that you are describing, it wasn’t noticing that I already had a paid plan whenever I tried to transfer THAT PAID PLAN. What did work was, from the My Sites page, clicking the Site Actions menu on the icon for the site that currently has the plan and following the obvious steps from there.