Changing the pro sites payment rule

Hi,

I want my users pay before use an upgraded plan.

For instance, an user subscribed to a plan PLATE, but in the same day he upgraded to a plan GOLD.

I want he use the GOLD features only after the plan price is debited.

It would be good if the price is debited in the same time in the user paypal account, even for upgrades.

How to do it?

Thanks.