UPDATE: You can now attach a role to an existing instance.

You need to update this lesson as you can now add roles to an existing EC2 instance.   Here is the documentation:

Posted On: Feb 9, 2017

You can now attach or replace an AWS Identity and Access Management (IAM) role to your existing Amazon EC2 instance. 

https://aws.amazon.com/about-aws/whats-new/2017/02/new-attach-an-iam-role-to-your-existing-amazon-ec2-instance/