AWS EC2 Instance pricing question

Hi,

You have details for the billing aspects of your upgrade here:

You can modify the instance size of a Reserved Instance if the following requirements are met.

Requirements
The platform is Linux/UNIX.

You must select another instance size in the same instance family (indicated by a letter, 
for example, T) and generation (indicated by a number, for example, 2).

For example, you can modify a Reserved Instance from t2.small to t2.large because 
they're both in the same T2 family and generation. But you can't modify a Reserved 
Instance from T2 to M2 or from T2 to T3, because in both these examples, the target 
instance family and generation are not the same as the original Reserved Instance.

You can modify the instance size of a Reserved Instance only if it supports instance 
size flexibility. For the list of Reserved Instances that don't support instance size flexibility, 
see Instance size flexibility.

You can't modify the instance size of Reserved Instances for t1.micro instances, 
because t1.micro has only one size.

The original and new Reserved Instance must have the same instance size footprint.

For Instance size flexibility mentioned above: see https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/apply_ri.html#ri-instance-size-flexibility

Best,

Didier

When you upgrade your EC2 Reserved Instances from m5.2xlarge to m5.4xlarge, the billing process will be a bit more complex due to the difference in instance sizes. Here's how it typically works:

1. Your existing m5.2xlarge Reserved Instances will continue to provide a discount for equivalent compute capacity. Each m5.4xlarge instance uses twice the capacity of an m5.2xlarge.

2. For the 5 instances you want to upgrade:

   • The discount from your existing m5.2xlarge Reserved Instances will apply to half of each m5.4xlarge instance's usage.
   • The remaining half of each m5.4xlarge instance's usage will be billed at On-Demand rates.

3. For the 4 remaining m5.2xlarge instances, you'll continue to receive the full Reserved Instance discount until they expire in September 2025.

4. After September 2025, when your original Reserved Instances expire, all instances will be billed at On-Demand rates unless you purchase new Reserved Instances.

It's important to note that while you've already paid upfront for the m5.2xlarge instances, the upgrade to m5.4xlarge will result in additional charges for the extra capacity you're using. To optimize your costs, you might want to consider purchasing additional Reserved Instances to cover the increased capacity of the m5.4xlarge instances.

Remember that AWS automatically applies the most beneficial pricing to your usage, so you don't need to manually allocate your Reserved Instances to specific instances. The system will automatically apply the discounts to eligible running instances.

To get a detailed view of how these changes affect your bill, you can use the AWS Billing and Cost Management console, particularly the Cost Explorer tool. This will help you analyze your savings and usage patterns after making the upgrade.

Sources
How billing works with Reserved Instances - Amazon Elastic Compute Cloud
Reserved Instances - Amazon EC2 Reserved Instances - AWS
Usage billing - Amazon EC2 Reserved Instances and Other AWS Reservation Models