Hi @DesignerDiana 
That is a good question.
Webflow prorates plan changes based on your existing balance, credits, and how many days you have left in your billing cycle.
For more information on this, please refer to this article: http://help.webflow.com/faq/what-happens-if-i-upgrade-or-downgrade-my-plan-in-the-middle-of-my-payment-period
Hope this helps