I have a light weight Application hosted on a Medium EC2 environment which i need for longer duration, While launching i opted for Ondemand instead of Reserved. How do i now convert a running OnDemand EC2 Instance to Reserved Instance?
All you have to do is to purchase a Reserved Instance of the same type as the current running instance.
They way this works is that AWS compares the current list of your reserved instances with the running ones, and will bill activity at the reserved instance price for a number of running instances equal to the number of your purchased reserved instances.
For example: you have 4 running c1.xlarge instances. You purchase 2 c1.xlarge reserved instances. From this moment on, AWS will bill activity this way: 2xc1.xlarge at the reserved price, 2xc1.xlarge at the on-demand price.
Hope this is clear enough, let me know in the comments.