Can I upgrade or change a savings plan?

If you're purchasing Compute Savings Plans - the discount applies automatically to any EC2 usage, or Fargate, or Lambda. If you start launching more resources, you can't edit your existing Savings Plan commitment, however you can purchase another commitment (another Savings Plan) to cover any remaining usage.

If you're purchasing EC2 Instance Savings Plans, because this one is a bit more restrictive, it will only apply to a specific instance family and generation within a specific AWS Region. But if you decide to launch more resources, you can again simply purchase additional Savings Plan to cover additional resources and still benefit from the discount.

I suggest to check the recommendations and reporting that is available for Savings Plans or Reserved Instances in AWS Cost Explorer - you can see based on your current usage if you get any recommendations for any extra Savings Plans: https://docs.aws.amazon.com/cost-management/latest/userguide/manage-sp.html