01 December 2015
There isn't actually any effort required in moving your on demand EC2 instance to a reserved instance. The only actual change is a billing change, you don't need to do anything in your instance configuration.
The only important thing to remember is that you're reserving an instance in a particular availability zone. The billing effect will only apply to instances launched in the same availability zone.
Amazon will apply the discount of having a reserved instance to your invoice automatically. They provide quite extensive documentation on reserved instances on their site (here). Tip