Users unable to upgrade subscriptions.

I previously established a free membership subscription and level, both named Silver. I'm ready to roll out paid levels/subscriptions of Gold and Platinum. However, there is currently no way for any of my users to updgrade.

The renew form for Silver members reads: You are currently on the subscription : Silver
Your membership is set to automatically renew

The subscription form for Silver members reads: 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.

I have searched and found some old forum threads with the same issue, and the reply is typically that no upgrade function currently exists, but is a planned feature. One suggestion is to allow memberships to expire, and only then may users upgrade. This is not an acceptable solution for me. I have also seen it suggested that upgrades may be possible with some custom configuration of the code. If anyone is aware of anybody that has successfully done this, or an idea of how it could be accomplished, please point me in the right direction.

As it stands, this plugin is worthless without upgrade functionality.