When upgrading your membership on the web, you only need to pay the price difference between the two plans. Once the payment is completed, you will immediately receive the membership credits associated with the upgraded plan. The next renewal date will be recalculated based on your upgrade date.
For example, if you subscribed to the Basic monthly plan on the 1st of this month and upgraded to the Plus monthly plan on the 15th, you will only pay the $20 difference and instantly receive 2,200 credits. Your next renewal for the Plus plan will occur on the 15th of the following month.