[Pro Sites] What happens when user on one subscription purchases something different?

What happens to the user's current subscription if they upgrade, downgrade or just change the subscription frequency? Are those passed on to PayPal so the existing subscription gets terminated and another one started? Or the price of their subscription changes but their due dates remain the same?
I assume user does not get credit for any unused time on their previous subscription, or do they?

Is the underlying plugin logic documented anywhere for this type of scenarios?

Thanks!