If you want to upgrade to a higher plan before the end of term, the remaining credit of your current plan will be applied to your upgrade. Once the credit expires, you will be charged for the cost of the new plan monthly/annually.
In the case of Agency Plan, you will be able to add additional websites. If you are paying annually, and six months of the year have passed, when you add a website, you will pay half of the annual price for each website. If an additional website costs $120, you will pay $60 for the remaining six months. The complete price of each additional website will be applied at the beginning of your annual or monthly billing cycle.
In the case of AppSumo, billing cycles are always annual. To update plan, you must apply a valid promo code. All AppSumo promo codes expire on December 3rd of 2019. Users of AppSumo can buy additional websites applying the same logic as the annual Agency plan.