Savings plan billing

If you have a savings plan that covers a c6i.4xl and you upgrade to the c6i.8xl, then the savings plan will cover half the cost of the c6i.8xl and the other half will be the on-demand price. When you return the instance to c6i.4xl, the savings plan will continue to cover it. Therefore, you will have an on-demand charge for the time the instance is a c6i.8xl, but the savings plan utilization should not change.

https://docs.aws.amazon.com/savingsplans/latest/userguide/sp-applying.html

It will replace the cost for 2 weeks in which you use upgraded size and cost will be according of upgraded size for 2 weeks and remaining cost will be according to downgraded size