When you upgrade your plan (to 'Pro' or 'Business'), you will pay the difference between the subscription fee of the new plan and the prorated amount you have already paid for the remaining time in your current billing cycle.
Upon payment confirmation, you will have access to the features of your new plan. Your access will last for the full duration of your new subscription term.
Example: You have subscribed for a plan that costs USD 250 per year. Then after 6 months you decide to upgrade to a new plan that costs USD 500 per year. You will pay USD 500 minus USD 125, which is the prorated amount you have already paid for the 6 remaining months in your current plan. So you should pay only USD 375 for your upgrade, and you will have access to your new plan for another full year.