How do I set up a subscription hierarchy?

I have three subscription plans Silver, Gold and Platinum. I want the users be able to upgrade from silver to Gold or platinum, but Gold or platinum members can't "downgrade" to Silver. In other words I need to setup a hierarchy that allows only upgrades, and disallow downgrades.
any ideas? Thank you in advance.

Hi David

Unfortunately, at the moment, we don't have a way to setup the system like that. In Membership Pro, subscribers can sign up for any subscription plans they want.

So in this case, if someone has signed up for Platinum, we have no way to prevent them from sign up again for Silver or Gold plan. This restriction / limitation is not implemented in Membership Pro

Maybe you can try to find a different way ?

Tuan