Let members upgrade

I have spent 45 minutes searching the forum but found nothing...

I have a membership site with 3 levels, visitor, free and member.

The free level can view some shortcodes and the member level can view everything on the site.
I have set Free as the default free subscription and it works just fine.

How does a user with the free subscription upgrade to a members subscription? When I try to access the subscription form when logged in as a free user I only get this:

Completed: Thank you for joining

It looks like you are already a member of our site. Thank you very much for your support.

If you are at this page because you would like to create another account, then please log out first.