Membership Upgrade?

Hi! My issue is simple. I have 2 membership levels; Test Drive and Full membership. The test drive (free) lasts for (let's say) 1 day. This gives them access to some things, more than a non-member, but obviously not the really good stuff.

How do I offer the member the option to upgrade right now to paid? It looks like once the membership is established when they go to the signup form, they are simply thanked, told they are already a member, and then suggested to log out if they want to create another account.