If you're finding that your site is a little less or more popular than you thought, or you need more or fewer sub-accounts, you may need to change plans. Login to ethnio, go to the top right-hand corner where it lists your account name, then Plan & Usage. Scroll down to "traffic & usage" to see how your numbers match to your plan.
With the free plan, you can always access data, but not create a new screener, and not activate a screener.
Ethnio will automatically prorate any changes to your account. For example, if you were to buy a "Little" plan and then upgrade to a "Huge" plan two weeks later, it would apply your credit minus those two weeks. Sweet. Screenshot below.