The payment system will recalculate the difference if you want to upgrade your plan on the day you have to pay

For example: If you have paid 297 dollars (Silver subscription plan) and you want to upgrade to 397 dollars (Silver+ subscription plan), then the system will add the difference on your next billing date

You can upgrade or downgrade multiple times per billing cycle.
Paypal (our credit card processor) will automatically charge the used package for the used amount of time.