Only moves subscription instead of adding it

Hi folks,

I need your help!

My system: I have 2 subscriptions. 1 free and indefinite, that is automatically added, when the user registers. 1 finite premium subscription, which the user has to pay for.

My Plan: The users always keeps the free sub, but can for a set period of time add the premium sub. In any case, the user always keeps his free sub.

My Problem: When the user orders the premium sub (and pays for it via PayPal), he loses his free sub. In other words, the free sub is moved to premium, instead of simply adding the premium sub.

What could I have missed?
Any ideas?