When you upgrade, you are purchasing a new package of credits. This means if you're currently on the 100 download credits plan and you upgrade to the 200 download credits plan, your account will receive 200 new credits added to your current balance (for a total of 300 that month). However, when your credit balance renews, it will renew with 200 credits only.
Please be aware, that all credits must be used during the month they are received. They do not 'roll-over' to the following month. However, if you do choose to upgrade, any newly assigned credits will be good for 30 days from purchase. So if you decide to upgrade to 200 download credits plan to make a purchase, you'll have an additional 30 days from the date of purchase to spend those credits.
If you've used all your credits for the month, and you prefer to purchase an additional product without using credits, simply add the product to your cart by selecting 'Add to Cart' (this will be at the full price). Once you have the item in your cart, you'll be able to check out using any payment method you choose.
You can find your total credit balance and their renewal date on your membership page.
How to upgrade your membership
How to downgrade your membership