Did you purchase RIs Working with Microsoft or RHEL?
Did you know, that under the service terms of AWS, you can find a section [5.4.1] that say that if any of them changed the pricing of their offering you may:
(b) convert your EC2 Reserved Instances with Windows or RHEL to comparable EC2 Reserved Instances with Linux,
or (c) terminate your EC2 Reserved Instances with Windows or RHEL and receive a pro-rata refund of the up-front fee you paid for the terminated EC2 Reserved Instances with Windows or RHEL.
Stuck with Windows RIs and want to convert to Linux?
Want a full refund of your RIs?
Just wait for a pricing change!
It's worth familiarizing yourself with the service terms, you can "play the game" better :)
https://aws.amazon.com/service-terms/