In order to change the instance type on EC2 Linux instances ENA needs to be enabled. In most cases this is already set, however if you have some older EC2’s running they will need to be ENA enabled for this change to occur. Otherwise you will need to create a new Instance from a fresh snapshot or detach and re-attach the EBS volume.

ENA is a custom network interface optimized to deliver high throughput and packet per second (PPS) performance, and consistently low latencies on EC2 instances. Using ENA, customers can utilize up to 20 Gbps of network bandwidth on certain EC2 instance types. ENA-based Enhanced Networking is currently supported on X1 instances, and will be available on other new EC2 instance types in the future.

Open Source licensed ENA drivers are currently available for Linux and Intel® Data Plane Development Kit (Intel® DPDK), and we will soon be releasing an ENA driver for Microsoft Windows® operating systems. The latest Amazon Linux AMI includes the ENA Linux driver support by default. ENA Linux driver source code is also available on github.com for developers to integrate in their AMIs. There is no additional fee to use ENA. For more information, read the Enhanced Networking documentation.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/enhanced-networking-ena.html

Testing Whether Enhanced Networking Is Enabled

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/enhanced-networking-ena.html#test-enhanced-networking-ena

ubuntu:~$ modinfo ena ERROR: modinfo: could not find module ena

Enabling Enhanced Networking on Ubuntu

The latest Ubuntu HVM AMIs have the module required for enhanced networking with ENA installed and have the required enaSupport attribute set. Therefore, if you launch an instance with the latest Ubuntu HVM AMI on a supported instance type, enhanced networking is already enabled for your instance.

modify-instance-attribute (AWS CLI):