Compute Savings Plans flexibility

Compute Savings Plans offer great flexibility when it comes to changing your EC2 instance types. If you've purchased a Compute Savings Plan for an EC2 machine and want to upgrade after 2 months, you can do so without any issues or manual intervention.

Here's how it would work:

1. Your original Compute Savings Plan commitment is based on a dollar amount per hour, not a specific instance type. This means you're committing to spend a certain amount on compute resources, regardless of the exact configuration.

2. When you upgrade your instance from 4 vCPU/8GB to 8 vCPU/16GB, the Compute Savings Plan will automatically apply to the new, larger instance.

3. The price variation will depend on how much your hourly usage increases with the upgraded instance. Your Savings Plan will cover usage up to your committed amount at the discounted rate.

4. If your new, larger instance's usage exceeds your original commitment, you'll pay the discounted Savings Plan rate for the committed portion and the regular On-Demand rate for any usage beyond that.

5. You don't need to modify your existing Savings Plan or purchase a new one to accommodate this change. The plan automatically adjusts to cover your new usage pattern.

This flexibility is one of the key advantages of Compute Savings Plans. You can freely change instance types, sizes, regions, and even move between EC2, Fargate, and Lambda while still benefiting from your committed discount. The plan will always apply to give you the best possible savings based on your current usage.

If you find that your new usage consistently exceeds your original commitment, you might consider purchasing an additional Savings Plan to cover the increased usage and maximize your savings.

Remember, Compute Savings Plans can provide up to 66% savings compared to On-Demand pricing, so it's generally beneficial to cover as much of your usage as possible with these plans.

Sources
EC2 Instance Savings Plan VS Flexible reserved instance | AWS re:Post
Compute Savings Plans – Amazon Web Services
Compute Savings Plan | AWS re:Post

All your instances will in-scope of a Compute Savings Plan. Go to https://aws.amazon.com/savingsplans/compute-pricing/ and set you region, term length, payment option, OS & tenancy to see what your saving will be on your particular instance type.

If you have more than one instance type, sort the table on the Savings over On-Demand column. The savings plan will go down through this in descending order, and will be applied to the instance type with the biggest saving first, then the second-biggest, and so on. Until either the money runs dry (and the rest of your usage will charged at the on-demand rate), or it runs out of instances to be applied to (in which case you've over-committed).