Final answer:
Reserved Instances is the most cost-effective option for a company that may need to change EC2 instance types.
Step-by-step explanation:
The most cost-effective instance purchasing option for a company that may need to change the EC2 instance type during the one-year period is Reserved Instances. Reserved Instances allow users to reserve capacity for a specified duration, and they offer a significant discount compared to On-Demand Instances. With Reserved Instances, the company can reserve instances for a one-year term and then modify the instance type if needed, while still benefiting from the discounted pricing.