Change assigned key pair on amazon ec2 instance

First create new key pair in Amazon.
then use puttykeygen to copy the public key, and also to generate private key from it (.ppk)
then just copy the public key to this location
/home/ubuntu/.ssh/authorized_keys

and before closing the ssh terminal, try to log in to the server using the new ppk file.

http://www.faultserver.com/q/answers-cant-change-assigned-key-pair-on-amazon-ec2-instance-310926.html