This article walks through the logistics of upgrading a subscription
Yes, if you upgrade your current plan, then you will be issued a pro-rated refund for the current plan and charged a new subscription for the new plan. For example, if you have been on the annual BASIC plan for six months and want to upgrade to the annual PRO plan, then you will be refunded $16.583333333333332 * 6 months = $99.49999999999998. And then, you will be charged a new annual subscription for the annual plan at full price.