Paid Subscription and Upgrade Process

The Subscription Upgrade process is not working as expected.

I've setup 3 subscription plans:
1) bronze (free)
2) silver (50$)
3) golden (100$)

When a user register his account he will get a free subscription. After the free subscription expires he can upgrade to a paid subscription. At this point a user buy a subscription, for example silver. But, after the paids the first subscription if he visits the "manage your subscription" page he is able to upgrade to another subscription (golden) without routing through paypal.

I need that a user with a silver plan can upgrade to golden plan only routing trough paypal and paying 100$.

And this should be a normal behaviour for a membership plugin.
Is something that I'm missing?