How to get membership upgrade to work?

I can't figure out how to get membership upgrades to work. I have two user member levels - a basic level, and a premium level. I have separate menu items and permissions for basic members and premium users. I have it set up so that basic members have a menu item to upgrade to premium. This menu item goes to the Subscription Form for premium level membership. The problem is that after a basic member subscribes to premium membership, they are still a basic member in addition to also now being a premium member. This means that they are seeing all the menu items and content for basic AND premium members, including all the content and menu items telling them to upgrade to premium membership.
I need a way to make it so that when Basic members subscribe to Premium level membership, they are automatically removed from Basic membership. The way it is now, they are both Basic and Premium members, so they are still being nagged to upgrade to Premium even though they are already a Premium member.

Hi Greg

The correct way for renewing membership is that users access to their subscription history page (did you create a menu item to link to subscription history page ?) ? From that page, he can access to his subscription record and process upgrade his membership. Did you try ?

Tuan

Maybe you can contact me via Skype, YM, MSN or Gtalk so that we can discuss more details about this and I can guide you how to setup it (you will also need to setup a upgrade rule first) .

Tuan