If you renew early, one of two things will happen:
If you have rollover enabled: You will be billed the full amount of your subscription + the rollover fee. Your total credits will be your subscription amount + the left over from the previous period.
If you do not have rollover enabled you will receive a prorated credit for the number of credits in the account. You will be billed your normal amount minus this proration, Your total amount of credits will be the same number as your normal subscription.
In both cases, early renewal resets your renewal date to be 30 days in the future.