When you upgrade your membership during an active subscription period, you will be charged the difference for the upgrade immediately and will be given the additional difference in credits. Your billing/renewal date will stay the same as it was previously and you will be automatically billed at a new subscription rate when the original renewal date is reached.
For example A user with a Standard Membership ($4.99 / 75 credits a month) with a renewal date of August 28th upgrades to a Premium Membership ($9.99 / 200 credits per month) on August 22nd. They will be billed the difference of $5.00 on August 22nd and also be given the additional 125 credits. They will now be billed $9.99 on August 28th and the 28th of each month for a Premium Subscription unless the user changes the subscription plan 72 hours before the renewal date.